Just because they use a gmail addy doesn't mean they are a scam or legitimate. While I agree with you that your own domain makes it more feel more legit they may be a broker just trying to make a piece themselves (nothing wrong with that).
THAT BEING SAID:
I NEVER (EVER) enter into private CPA relationships. I have found over the course of running my site for a few years that for whatever reason (read: my fault for not writing more posts directed towards it) I don't do well with CPAs and if I am not going to do well AND I CAN'T KEEP TRACK of "the action" part of it – its not worth the headache to give up that real estate.
Whenever I receive one of these offers I do 2 things:
1) Try to turn it around on them. Tell them to keep their money, ALL of it, and just pay for the spot. Something along the lines of "I am not comfortable with private CPA relationships so keep the money associated with the sale and pay X for the month."
and
2) Ask why they won't work with CJ or FlexOffers? Usually they give you some bullshit answer that they want to give you more money. I always respond that I don't want more money I just want to jump behind a company I tend to trust.
It is all about risk. They are taking ZERO risk when you put up a CPA ad, and you are taking all of it…let them put their money where their mouth is.
I should mention that there are probably a ton of bloggers that make a ton of money using private CPAs I just don't feel comfortable with them