"Trust" and "transfer" are issues for buying and selling anything online. Without a trusted intermediary (like an escrow company), either the buyer or the seller must assume a weak position and be forced to trust the other party.
Often times, escrow companies require casheir's checks to make the transaction happen faster. Also, from the seller's perspective, waiting for a personal check to clear vs accepting a cashier's check are about the same thing.
Without transacting a lot online, it can be hard to know whether you're being overly cautious, or whether your being taken advantage of.