This question is about integrity vs. greed, not capitalism.
If you were in the business of selling gel seats for a living, then you charge what the market will bear... or discount to outsell your competitors. That's capitalism, just business.
If you just have one gel seat to sell, it's a matter of how you conduct yourself in your personal life, and interaction with others.
You have two choices...
1. Sell the seat for more than what you paid for it, take the guy's money and walk away. The guy you sold it to knows you took advantage of the fact they are hard to find, and probably resents it. Yeah, you made a few bucks, so I guess the money is pretty important to you.
2. You sell the gel seat for what you paid for it, or even less if it's a little worn. The guy realizes you could have made more money, but he shakes your hand, looks you in the eye, and you know you just gained another man's respect. You walk away feeling good, and maybe you made a new friend.
Even if the guy thinks you're a sucker for not charging more, that's his problem. You know you did the honorable thing and you're integrity remains strong.
I choose #2. A few extra bucks isn't worth losing even a small piece of my character, which I try my best to uphold to ethical standards. I guess it was my Grandad who pounded it into me... "You can lose your money or your house, and still build a new life. But if you lose your honor, you lose everything that makes you a man."