Thanks everyone
The reason I replied to it is because it wasn't an email that arrived in my inbox, it came through a form on my website. So whoever this was took the time to search my site for the contact form, then went ahead and messaged me that way. I immediately delete these sort of things that come straight into my inbox!
Anyway, to update, I just wrote him back and said after some consideration, I will not be able to go through with the transaction. I haven't heard from him since, and I hope that's the end of that!