There may be no buyer, and that is the honest starting point
Selling an unlisted share means finding somebody who wants to buy it at a price you will accept. There is no exchange, so there is no standing bid and no guaranteed counterparty.
No timeframe can be promised for an exit, and you should treat any that is promised to you with suspicion. How long a sale takes depends on whether there is demand for that company at that moment, which is not something a platform controls.
This is the risk that most often surprises people, because it is invisible while the position is only being held. It arrives at the moment they want the money.
What a sale involves
A price is negotiated, as it was on the way in. The level shown on a company page is indicative and is not a bid.
You lodge a Delivery Instruction Slip with your depository transferring the shares to the buyer's account, against payment. Your broker executes that instruction; check their charges for an off-market transfer before you commit to a price.
Capital gains are yours to report. See the tax guide for how gains on unlisted shares are treated.
If the company has listed since you bought
A lock-in may apply to shares acquired before the offering, during which they cannot be sold at all.
After any lock-in ends, shares held in demat can be sold on the exchange like any other listed holding — which is a materially easier exit than the one described above, and is the point at which this asset stops being illiquid.