I'm pretty sure the feeling of being a chump (for me at least) is a social response to someone having gotten more out of you than you needed to give them, rather than a response to being unhappy with the exchange.

I sometimes get it when I retrospectively discover I paid more for something than I needed to, but it's at its strongest when there's an actual agent involved who I can point to and say "you, you took more money from me than you needed to, you scoundrel!" If I buy something online, when I could've gotten it somewhat cheaper on a different site, I just shrug it off.