You can offer any service you like here. Art, stamp making, toy hexing, etc.
If you're hosting a service...
Be clear about what you will be offering, anything you won't do, and what you expect in return.
Charging money for commissions is fine.
If you're soliciting work...
Follow the original poster's rules, and be courteous.
Both parties should exchange their end of the trade as promptly as possible (when applicable).