The shares in a Mercia EIS portfolio are illiquid, as they are in unquoted companies and therefor there is no secondary market.
Investors are reliant upon Mercia to sell the shares in the portfolio company to gain liquidity. The shares can be sold to another company, in either an acquisition or as part of a secondary acquisition. Alternatively, a EIS portfolio company may IPO, but at this point the shares are likely to be locked in for a period, before they can be sold.
In some rare cases there may be an option to sell shares in some specific companies in a very limited secondary market, but there will not be an option to sell all the shares in a fund at any point in time.
This illiquidity is inherent with all EIS funds, and is in contrast to the ability to sell shares within a VCT. However, the illiquid nature of EIS shares is the reason that they have such strong tax reliefs.