I don't see why this shouldn't be allowed since the merit system isn't fair and your told there is no need for you to keep this and you are encouraged to give them away, why shouldn't a person decide to sell them if they wish?
Because it goes against the very integrity of why the Merits were created in the first place. While its not illegal (you wont get banned for it), many DT members will red tag you for trying to game the merit system if you try and blatantly sell them.
And for the record, I totally think the merit system is fair. The ones that say that it isnt generally only shitpost and dont actually contribute to any dialogue carried out on the forums, IMO.