How about adopting a similar approach as DP? Forget seperating 'Looking For / Wanting to Sell Products' and 'Services Offered / Looking to Hire' and make it one combined forum 'Buy, Sell and Trade' (or something alike). Then have the sub-forums for all the different categories (design, sites etc) and you'll have a more streamlined, straightforward marketplace in my eyes.

You won't need labelling then but can still encourage it, after a while people should start picking it up if a majority are using it. Could also add a noticeable message on the New Thread page regarding the different types of label and how it's beneficial.