But when I see a for-profit script I know that the creator has an incentive to keep pushing updates and maintanance and design.
Also, you should sell your script yourself AS WELL as selling it on CodeCanyon. That way you can funnel people towards your site and make more money per sale PLUS get the benefits of being on CodeCanyon's community.
The internet is NOT Walmart, there's no exclusivity contract, you don't have to sell your product or service in only one place.