The problems I have with that (and I have been on ebay for 11 years now and completed more than 2000 transactions, 75% of them buying rather than selling) :

a. It is against the rules. You cannot circumvent the process (and the fees it implies). If you are a seller and want to put a BIN you do it. If you are a buyer and there is no a BIN, assume that there is a good reason for that. (would you go to your grocery store and haggle over the price of a dozen eggs? Nope.)
b. It is plain unfair (and unethical) against other bidders and bad business practice (not to mention bad karma) for sellers to complete transactions outside of the ebay process and leave legitimate bidders/prospective buyers out to hang. Sellers that do that directly get in my blacklist. If you list an auction as a seller, you sign a contract that you accept the playground. Same if you are buying something on ebay. If you do not like the playground, there are other places to hang, but there is no need to litter.