What ya think? Ppl can trade their high level hero for gems (reduce might) or dups.
If the seller could set the price then it would allow any player to create multiple accounts, use their free gems and trade all of the legendary Hero's to one account/sell off the rest to earn more gems.
Personally I don't think this would ever work. At a push they might have to set a price of the gems to 5%+/- a base rate to avoid the above. That being said the base rate would have to be rather high in the first place as to avoid making 'random rolls' redundant.
The same concept applies.
The 'seller' uses 500 HB's to get a random Hero and then sells it for all of the free gems that their new account has. I'm not exactly sure how many gems a new account gets but do a few free offers, remove some trees and rocks, get an achievement or two and you'll be up in the 500-1000 range. Rinse and repeat until you have enough gems to buy all of the Hero's that you desire.