Only killer is shipping. It is easy to go on a Yamatoku buying spree, winning CIB games for literally spare change up to maybe a few bucks, but their combined shipping fee jumps the total way up. Still can be a good deal, but shipping charges from Japan, be they real or inflated to make up for the dirt cheap sales prices, can screw you over in the end.
Not a system, but one thing to look for from Yahoo are Capcom arcade I/O boards.