It's good protocol to say 'I'll take it' and have this accepted on a thread.
However, there are always going to be exceptions to this and at the end of the day the seller should be free to sell his goods to who he likes - for example if a close neighbor pops up on the thread and PM's, the seller might want to sell to him as it's more convenient/avoids postage etc. A buyer with a known history of reliability might take precedence, too.
Finally, the seller shouldn't have to sell to someone he doesn't want to sell to - he might not like them, think some of his posts might imply irresponsibility etc.