To be perfectly honest, I have heard of selling and sponsoring not only tweets, but pins, likes, shares, you name it. In conjunction with that last statement, I would not mind selling tweets or anything of the like at all, as long as I know the promotion in question is something some of my followers will find useful. It may even be safe - in terms of not losing followers - and profitable for me since I have different twitter accounts specifically for each niche.
BuySellAds is once platform where this occurs, jjmyles, but you can come up with even more by searching "sell sponsored tweets" or anything to that effect with not only tweets, but any one up for a social network you can think of (provided you have enough people connected to you on them).