Unfortunately with auctions many people get emotionally attached to what they want. A stamp that may normally go for, let's say $50, can be driven up because someone just really wants it. I have been to auctions where people pay above full retail, plus taxes, plus auction fees.
It could be the person was willing to pay the same as you but then figured "What's another $5?"
When I go to auctions I look at things as "Can I turn a profit on this?" attitude. I may want to keep it myself but I look at the long term if I can. When I hang this up can I recoup my cost.