If the purpose is to demonstrate ownership of the address, wouldn't you want him to demonstrate the ability to
receive at the address? Anyone can send e-mail with any "from" address they want.
Yeah, I meant talk on it to verify but yeah just getting one from that address would work. I don't need any right now but if anyone else is looking to buy and he can email you from that address just to verify it then I'd say go for it. Should be 99.9% safe, the site is reputable from people I've talked to.