It's a warning from eBay, sent through the eBay mail system. I got a stack of these. It appears they are trying to target people who try to sell things outside of eBay using information collected from bidders. I must have been contacted by someone doing this, because I don't.

What it makes me believe is that they monitor the number of Contact Member requests that each of us makes.

If someone has infested the internal eBay mail system, it's really getting scary.