Yeah, I agree that if you list something for $150, and a buyer agrees to your price, then it's settled.
Frankly, at this point, I doubt anyone would be DUMB ENOUGH to publically bid up the $175 any further! So, even if you do end up selling @ $175, do you really want to be stigmatized for a lousy $25 more?
Finally, next time around, sell your stuff on Ebay if you prefer the bid approach. That's what I do.