I believe this is a very common eBay scam. Actually, these people also cruise internet "classifieds" looking for items for sale, agree to "purchase" the item, and use this same scam. I have heard several versions of it, but it always is roughly the same. Someone wants to "buy" what you have, sends the "Cashier's Check" for way more than the agreed on price (or winning eBay bid), with various excuses and explanations for this unusual action, then has you "deduct" the actual selling price and "wire" the remaining money.
The result is always the same. Typically, these people working on behalf of their "clients" are based elsewhere...Africa, Indonesia, etc.
Good job continuing to spread the word. The more people who are familiar with this type of scam, the less likely they are to be successful.