I've seen people post commissions for on site items and silver, but I was wondering if I could advertise my art here and ask in return real money. 

We don't prohibit this but you may do this only at your own risk.

I would suggest to use resources that are specialized on sales specifics and have a clear code of laws and rules as well as coded framework for guaranteed safety of such transactions. Unlike the situation with in-game goods, we don't have any means to control the both parties and the facts of payment made and art transferred to a new owner. We won't be liable for any transaction violations and we won't act as third-party judge in any conflicts that involve any currency or goods that aren't a part of BeastKeeper site. In other words, the situation is the same with the off-site trading.

I would recommend using a site that has a system set up to ensure payment... something like deviantart, or etsy, or other sites (Artbreak, Yessy, and Artpal are a few I have heard of, though I would do your research on them first). Third party sites usually have payment services (like paypal) al set up. Though if you are comfortable doing it on your own then I guess you would not need that so much (I've heard some people usually ask for a deposit). As Angel said, it would all be at your own risk. But advertising is always good! I have been on other sites where users utilize the community as a base to advertise their art galleries, commissions and sales. Having a gallery or website for people to look through is very useful. :)

Good luck!