my idea is to put all offers for puzzles with a set prize on the same place.

like these examples:
http://twistypuzzles.com/forum/viewtopic.php?f=4&t=13066
http://twistypuzzles.com/forum/viewtopic.php?f=4&t=12742
http://twistypuzzles.com/forum/viewtopic.php?f=4&t=12599
http://twistypuzzles.com/forum/viewtopic.php?f=4&t=12072

it would be simpler if they all was on a list containing the most inportant information, both for the seller and the ones who want to by one.
and if you click on one of them you will go to the offer.
if the offer is closed then it will disappear from the list

i made an example on how it could look

There are a number of sites that have this sort of thing. Bedardpuzzles.com is one such place. There are many more. Dave Calzone and I are working on one ourselves.

The problem I see with your idea is that there may be 3 people selling the same puzzle, and if someone doesn't know better, they'll think it's like the shelves at Wal-mart. Go for the cheapest, what the heck... Without trying to sound condescending, there are a number of builders who take someone's idea (even if it's not an original idea, but one that's currently being sold), build a lesser quality version of it, and try to pawn it off as if it's the same puzzle. If all of these builders/puzzles are advertised in the same place, people may be inclined to associate them together, and think they're the same quality.