i dont think previous bumps should be deleted, purely because iv seen several sellers lately abusing that. One seller, who will remain unnamed, bumped his thread at least 3 times in one day that i saw, by deleting his previous bumps
I usually bump once every 2 days, but i only do this because if in dont it just dissapears.... if everyone agreed to bump once a week or so, itl be a much less crowded place to be.