In my opinion, the reason why namepros might decrease retail value is simple. Domains on namepros will usually be listed for prices lower than the ones you would pitch to an end user. Someone might come in and see your domain sitting there unsold, bumped many times for a price lower than he is even willing to pay.
Even if you list your domain here for its retail price (which happens a lot), no one from the forum is going to buy it. Therefore, it will result in pages of bumps without a sale, which might discourage a potential end user.
There are workarounds to this, like deleting and making a new thread each time you want to bump your domain name but then it won't have time to get indexed in google, thus losing out on namepros traffic, which is the entire point.
All in all, I see no reason why one should post a domain which he intends to sell at retail price on namepros.