This stuff kinda works like ebay i guess. If you have a nitch , time and talent, there is money to be made here.
Someone needs a service / product you bid on being the supplier.
You need a part or a product made and the rest of the world bids to get your product made.
I never new this existed but my brother in law works as a CNC guy and his company is racking in the PO's.
How about some of that one off stuff like adaptors for car stuff ect. AEI here in Fairlea Wv makes small / large order stuff and partites on this site.
http://www.mfg.com