Colin...I get a lot of offers from websites too and some I follow up on...but none so far have given me the kind of exposure for free or the kind of service offered by Art-Agent for 5% commission. I doubt you'll find anyone out there with the quality of artist nation and art-agent!
However, check out their website..read all the fine print. E-mail them with your questions and doubts...see if you get answers or just more double talk. I also like to e-mail their online artists and ask for their recommendation. Sometimes I get all of these returned...unable to deliver. That makes me wonder, too. and sometimes the site posts no information to contact the artists. I did a search recently on the artists for one site and found that, although the site does not post contact information...the artists are professional, some with websites and several were mentioned in national art organizations.
It's good to know all you can about an online offer...as in any business proposition....and if you have doubts...forget it and move on. There will be another offer next week!