Selling shares · Castelion

How to sell your Castelion stock

Hold vested shares, options or RSUs in Castelion (defense tech, United States)? Here's how a private secondary sale works, the US rules that apply, who buys, and what to expect.

Sector defense tech · HQ United States · Status Privately held · Reviewed November 2026

Castelion is a private US defense tech company. It develops low-cost hypersonic weapons and missile systems. If you hold vested equity in Castelion and want to turn part of your Castelion stock into cash before an IPO or acquisition, this is a plain-English guide to how a private secondary sale actually works — and what's specific to Castelion and to US rules.

Independent guide. PrivateTechShares is not affiliated with, endorsed by, or connected to Castelion. This is general educational information for people who already hold Castelion shares, options or RSUs. Always check your own equity documents and take professional advice. We do not recommend or endorse any transaction: whether to buy or sell, and on what terms, is entirely the individual decision and responsibility of the buyer and seller, based on their own due diligence. What we do: we help you prepare — a clear picture of what you hold, what it may be worth, and the paperwork — so that when you decide to act, you are ready.

What's it worth?A realistic range for your Castelion stake in seconds.
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Hold options?See what exercising costs — plus the AMT hit.
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Getting ready to decide?Prepare properly with the toolkit — or a report run on your own numbers.
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Can you sell Castelion shares before an exit?

Often, yes. Selling existing shares you own in a private company to a private buyer, before the company is sold or lists, is called a secondary sale. It's possible for Castelion shares if they're vested and transferable, and once you've cleared Castelion's transfer rules. There's no public market for a private company, so a sale is a private, negotiated transfer — not the click of a button.

What you probably hold

First, pin down what you actually have. US startup equity is usually ISOs or NSOs (options you exercise into shares by paying the strike price), RSUs (which typically settle into shares on a liquidity event or set date), or already-exercised common shares. You can generally only sell settled shares, so if you hold options you usually exercise first — and exercising can trigger tax (including AMT on ISOs) before you have any cash. Check your grant for your vested amount, strike price, any 83(b) election and deadlines.

The rules that apply in the United States

How a sale works, and how it's taxed, depends on US rules — transfer formalities, any right of first refusal and co-sale rights, and the tax on exercising and selling (capital gains, AMT, and possibly the QSBS exclusion). We've written a dedicated plain-English guide: selling startup shares in the United States. Read it alongside this page.

Who buys Castelion shares

There are a few realistic buyers, roughly in order: Castelion itself (if it runs a buyback or tender), existing investors or shareholders (often via a right of first refusal), the investor in Castelion's next funding round, and — when those don't apply — a private individual investor who wants exposure to Castelion. Recognisable names like Castelion tend to attract private-investor interest — but the hard part for a holder is understanding what you actually own, what it might be worth and how a private transfer works. That's what our guides and toolkit are for.

Where holders actually find buyers → A plain-English map of every route — company buybacks, tender offers, secondary platforms and investor networks — with a neutral directory for the US and Europe.

What to expect on price

A private, minority, all-cash secondary usually happens at a discount to Castelion's last headline valuation — because the buyer takes an illiquid stake with no guaranteed exit, and because you likely hold common shares, worth less than investors' preferred shares. That's normal, not a trick. See what discount is fair for a realistic range.

For holders

Holding Castelion equity? Get the occasional, no-noise update when something changes that could affect your position — we watch the private market so you don’t have to.

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Thinking about your Castelion equity?

Exercising, holding or selling equity is usually a one-way decision. Doing it well takes a real assessment — what you hold, what it's realistically worth, the transfer rules that apply, and the tax. The equity toolkit (US edition) is exactly that homework in one place: the free tools give you the numbers; the toolkit turns them into a decision you can stand behind.

Get the equity toolkit — $39

US edition · instant PDF, no sign-up · EU edition →

Hold options and weighing whether to exercise? The ISO/AMT exercise calculator and the QSBS calculator can help you put a number on it.

Questions people ask

Can I sell my Castelion shares before an exit?

Often yes. If you hold vested, transferable shares in Castelion you can sell them to a private buyer in a secondary before an IPO or acquisition, subject to Castelion's transfer rules — typically a right of first refusal, co-sale rights and board or company approval. If you hold options or RSUs rather than settled shares, there are extra steps first. Castelion is a private company, so there is no public market; a sale is a private, negotiated transfer.

How much are my Castelion shares worth?

There is no live price for a private company like Castelion. A rough estimate starts from the price per share at its most recent funding round or 409A valuation, adjusted for your share class (common shares are usually worth less than the preferred shares investors hold) and a discount for a private, minority, illiquid stake. Use our free valuation estimator as a starting point — it is only an estimate.

Who buys Castelion shares?

Typically one of: Castelion itself (via a buyback or tender, if it runs one), existing investors or shareholders (often via a right of first refusal), the investor in an upcoming round, or a private individual investor seeking exposure to Castelion. Well-known names attract private-investor interest, which is why it helps to understand what you hold and how a private transfer works before you start.

Do I need Castelion's approval to sell my shares?

Almost always, yes. Castelion, like most private companies, controls share transfers through a right of first refusal, transfer restrictions and board or company approval written into your equity documents. No marketplace or buyer can override that. Start by reading your grant and shareholder agreements and asking Castelion's stock administration what is permitted.

What tax will I pay when I sell Castelion shares or stock?

It depends on where you live and what you hold. In the United States, selling settled shares usually triggers capital-gains tax, and exercising options first can create a separate taxable event (including AMT on ISOs). In Europe the treatment varies by country. This is general information, not tax advice — confirm your own position with a qualified adviser before you sell.

More about Castelion: castelion.com ↗ (official site) · funding & valuation data ↗. We're not affiliated with Castelion — these are neutral references for your own research.