I can speak from the perspective of the eBay API - the only way to get the username is if you yourself are the seller.  There may be other devious tricks that somebody will pipe up with, but programatically, eBay has obscured it.

Foul

Other than playing spot the pattern and trawl the feedback, the easiest option is still usually the "token bid" approach. Place a no-hoper bid at auction start and insert your username to your finished auctions list at the end to see who won;

cgi6.ebay.com/ws/eBayISAPI.dll?ViewBidItems&all=1&_rdc=1&userid=YOURUSERNAME&&rows=200&completed=1&sort=3&guest=1

It doesn't work for all sales - exclusions for private sales, high value sales depending on context, etc., but can be useful - kidzjesman on that recent 2nd print Chainmail, a very familiar alt on the Minotaur just before, etc. Some rather surprising OCE buyers, too.