in banks if you deposit a bad check/mo, you won't get the money (your deposit will be reversed). your bank does the depositing for you for free and you must take the risk - or wait long enough to ensure money is there.
your ebay seller must be a con man, or someone else has stolen the mo. a normal person will not use check cashier. they will use their bank. only the low class people use the check cashing services.
anyway someone gained from this scam - he got the money from the cashier w/o shipping the item to you. so there must be someone else loosing the money. now it is the cashier. they are trying to recover the moeny from you but I don't think they have basis to get it.
btw, how did they send you the letter? by certified mail? if just the regular 1st class mail then everyone can say I never get the letter.... and if it is 1st class mail I figure they were just bluffing.