If you ever need to sell all of your altcoins (bitcoins, ethereum, litecoins, etc) in exchange for fiat currency you definitely need to find someone who is willing to buy. That is the only way to “cash out” (if that is what you are asking).
In case there is none in your network (assuming you need to sell immediately your altcoins for fiat and at that time there is none offering to buy), there are other options, like transferring your funds to other markets or selling to friends willing to buy.
Your funds are not bound in any way to bitsquare, except if you have open offers in which case the amount you put up for sale plus some fees are locked for as long as the offer is open. If you decide to cancel the offer your funds are unlocked and remain in your personal bitsquare wallet for as long as you want them there. Whenever you want you can withdraw your funds to another wallet independent of bitsquare.