How long should I wait to accept/decline offers on Neopets?

I have an item up for sale on Neopets, and two people have placed bids. One is higher than the other (the second one) and they are giving me more than what I asked for as the other (first bid) gave me almost exactly what I asked. Should I wait longer to see how high it goes? I have never used the trading post before so I do not know how it works O.o