that happens all the time for my other hobby, Macross. usually a group buy is easier to save on shipping/effort as opposed to shipping items to two different addresses.
the more common shady that happens on Macross World buy/sell/trade thread is silent bids: even though it clearly states it's prohibited, you must list a price and no ebay links to other sales.
one guys offers it at a specific price, you pm him and agree to it, he reads the pm but goes ninja saying he was away for the weekend. the another guy pm's. he responds he has an offer already fishing for a response for a higher offer. since Macross stuff is so rare impulse buyers automatically will offer more.
but the group buys seems more out of convenience but it's kinda wack if he already agreed to a sale then backed out.
i try to go by the order of PM's received but i tell them if they don't respond by x amount of time i'm going to offer it to the next pm.
i do that bc sometimes if they give you a best offer and you accept it they will take it and shop around to see if they can get even a better offer saying : this guys is willing to sell it at x price, can you go lower?
it's such nonsense but i got used to it.