AltCoins or any crypto currency can be traded for another coin of your choice as this case maybe.
If you're also being in doubt that may just end up holding and stocking invaluable coin then you can earlier on be concious and only Invest on populous coin that trading and exchanging for fiats wouldn't be a dump asset to you. Prevention is usually better than to cure.
actually this is not what OP is asking instead the word "WHAT IF" is the thing here,
that he is asking about the coin that after you bought and when you decide to sell there are
no one that is buying and for me this is more than impossible to happen because unless the coin
is delisted in a certain exchange or being blocked by the government ? there is no way that
you will not find buyers of your coin.