Wait, did you gave them your money already?
It's not usual, but not seldom either. I've never made a deal with an artist. You should be careful with whoever you're talking with. Remember the internet rule: Everyone is a stranger. They do not know your motives, you do not know theirs.
Well, if you're always updating your novels, they're just watching on the sides, waiting for you to take the bait.
Just like when I updated Paulina Rex. I haven't done that for a year, but when I did last January, behold, I got an offer for a cover art.
Kinda annoying too, coz they PM me like this,
"I make better cover than what you currently have."
And I was like, "Mfker, that's my illustration you're insulting."