You're not selling your account so they can datamine reddit. You're selling it so that they can put ads on Reddit that look like a user commenting. Which is also a thing Reddit does.
There are heaps of vendors. Just google buy and sell reddit accounts. (Maybe use a better search engine like duckduckgo, I'm not sure actual google will bring you to those sites).
It is if its on the Reddit platform. The more dmaage done before their IPO, the better. Spiteful and petty? Absolutely. But because Steve Huffman is the target? Let it burn. Actually, everyone should help pour some fuel on it.
I think there is no way to delete Reddit PM's and chats. So the buyer would get your possibly personal correspondence plus whatever private things that other people wrote to you. That can't be good. I am pretty sure I never doxxed myself in a PM, but other people surely did. I always tried to move discussions from PM to email or self-hosted chat, but people tended to refuse. They trusted Reddit, the silly billies.
Frankly if it weren't the same username I'd have sold my account already. Reddit shows no loyalty to those who made it the largest forum on the Internet, they deserve no loyalty back.
Personally I'd prefer not to have someone speaking for me, despite the supposed anonymity of the accounts. If you post any amount of content, it's likely the account can be linked back to you after the sale, which could prove problematic depending on what the new owner does with it.
But I'm a bit paranoid about such things after experiencing internet stalking. I don't see anything morally wrong with it, just that consequences from the sale might affect me negatively in the future.
It's probably against their TOS, so they could ban the account. It's not illegal. I would say it's pretty gray morally, since it would probably be bought by an entity that will pretend to be a real person to sell product or spread misinformation.
I don't see anything wrong with it. You made that account and built it up. I don't have a reddit account, but if I did, I assume I'd somehow get scammed during the selling process and not get any money.