Im a seller, I do that too. Some people believe that as soon as the seller receives your payment they should leave you positive feedback I disagree.
I used to leave feedback upon receiving money but I have come to realise that because the buyer has paid that doesn't mean the transaction has finished and there heaps of room for them to be a bad buyer. Eg. Someone buys something from me, pays me by cheque, in my auction it states no pickup, then the buyer rocks up unexpectedly at my door at 9pm wanting to pick up his parcel. That is a bad buyer, even though he has paid and had I left him good feedback I would have no recourse and be unable to warn other seller not to do business with him.
Having said that, if someone were to leave me legitimate negative feedback I wouldn't give him retalitory negative feedback just to be a bitch. A lot of people won't do this though and they sort of blackmail you into not leaving bad feedback through fear that they wil ruin your perfect rating.
What I now do is I have 2 accounts, one for selling, one for buying. As a seller its important for me to keep my 100% rating and I dont want to buy something from someone dodgy, leave bad feedback and get rtalitory feedback, so all my purchases are made in my other account where I am not concerned if they neg. me because as a buyer its not as important to have that 100% rating.