I think you're both right. I believe it is a seller's market because of the new playoff addition which makes more playoff spots, and as such more teams that think they are in contention, which makes more buyers than sellers at the deadline. (Teams really have to suck in order to be out of it by now to be sellers) I would also argue that teams that are buying for the playoffs/championship are more desperate to improve than the sellers are desperate to sell. (In other words I think not doing the deal is more harmful to the buyer than the seller)
That said, SR is right that as the deadline approaches, it evens out because the sellers have to lower their demands or risk not doing a deal and ending up with nothing for their players. Although theoretically they could still hold onto a majority of their players with qualifying offers or get draft picks, right?