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