I don't know whether the term 'direct' works with such one off, higher priced items like paintings. IMO when it comes to an investment in original art, people like to view the work in person before committing to buy, as you don't always get an accurate impression online.
There are quite a few online art galleries out there, allowing artists to list their work, for free, with a commission on any sales, the obstacle is getting the artists to list their works with you over another site. Another problem is screening for quality in what is being listed.
Looking at the(now dufunct) .com site, they seemed to have run quite a successful site for a few years.