You may want to offer more than 3 dbs, depending on which player you are after. Usually the player needs to be killed more than 10 times to get a head. I would offer at least 10 diamond blocks. But if you were just buying a random head, 3 dbs would be enough.

@HaloNest Quite sure you want my head

If that's the case, considering that you are an experienced player and will not likely accept random TPA requests that could get you killed, if I wanted your head specifically I would pay at least 20 dbs for it.