It's not associated per se
, but rep is a great way to show that you are a trusted, helpful, and respected member of the forums; therefore it is assumed you can be a responsible seller. It's just to protect the members against scam/fraud, as if we didn't have a minimum rep count, people could just sign up and post sales.
Think of it as a way of moderation, it may seem irritating at first, but it's a preventative meaure, and assures that the person you are buying from is legit.
The iTrader ratings themselves are just like eBay feedback really. Also, note that you only need 35 reps to sell, or post in the wanted section. You can buy things as soon as you sign up.
There should be a little link at the top of the page to the Rules section, if I remember. (on my phone right now).