Hi, it has occurred to me when selling an account, that two people bought from the same middleman link. The middleman then made two tickets and I had to explain to one buyer that I couldn't give him the account because another one bought it first (the tickets also had a big time difference). And on top of that cancelling the ticket and placing the second buyer to another wait, so he can get his money back. What I am proposing, is an option to auto-lock the middleman link after a certain amount of purchases (for example 1, if I'm only selling 1 account through that link), so that the product is no longer available. This would probably add additional work when an order is cancelled and the link-lock threshold is reached (though I suppose re-opening the link could be automated aswell), but it's certainly better than having the middleman open multiple tickets for a product that was intended to be sold only once.
I definitely agree this needs to be fixed but we're planning some bigger things with the middleman system and this is going to mandatory be implemented in those updates. So in short, it's approved your suggestion but it won't be released until the middleman system is partially redone which is in the works already.