Honestly...I think you are wasting your time. I've been to Cancun twice and each time...the dollar is king. It's a nice idea, but I really don't envision the shop keepers wanting to trade t-shirts for knives, and I am not quite sure they would know or even believe the claims of quality...but this is just my opinion.
I've been to the downtown market twice..I won't go back. It's crowded and seems to be an endless maze of small shops, each one pretty much containing the same thing as the last. We left one store, only to confronted by the next shop keeper who, no lie, said, "Come see my crap!" very enthusiastic, but not want you want to hear. If you go in one store, they will hound you to visit theirs, not enjoyable..at least for me.
Just flashing dollars can get you a deal. One store I wanted 4 shirts to take back..so I pretty much figured up how much I wanted to spend, got out the appropriate amount of $ and just told the guy, "I will give you this for 4 of those"..and he took it. Now think of the explaining you'll have to do in trying to trade the knife..just a thought.
Cancun was gorgeous, but not sure how it will look after the last hurricane..they got slammed pretty hard. I recommend the all-inclusive route. You can eat and drink all most 24 hours a day, very secure and awesome pools and beaches. I've stayed at Cancun Palace and can't remember the name of the other, have to look it up.
Have a great time and just remember to hold on if you take the bus into the city from Hotel Row...these guys drive like mad! and make sure you visit Senor Frogs..fun time.
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Get busy living...or get busy dying!