As Knives already said, generally the principle is that you bid a junk item worth next to nothing along with the amount of nps they've requested to bid "pure nps". To transfer nps to another account you'll probably either have to use the Trading Post or your Shops, just place an item in one of the two in the account you are transferring the nps to (don't forget to price them if you've put it in shops, usually the idea is to way overprice), from there just go into the account you're transferring nps from and buy the item/bid on the Trade