This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
You usually avoid unlimited liability by placing a stop order to cover your position as soon as the price goes sufficiently high. Or for instance you can bound your losses by including a term in the contract which says that instead of giving back the stock you borrowed and sold, you can pay a certain price.
Note that for volatile assets (the very ones where you feel uncomfortable about unbounded risk), stop orders are not guaranteed to help. Remember, prices are not continuous - there is a discrete sequence of bids. Price can go from below your stop to MASSIVELY above it before your stop order can be executed. Most often this happens on news when a market is closed, but it can occur intraday as well.