I do not see why it cannot be posted in Sell / Buy / Auction board.
[trading form]->[main characteristic]
Buy/Sell -> need money
Auction -> use of bids
Barter -> no money
What next? "Trading" and "Stealing"?
Was this needed...?
What's more, buy/sell/auction/barter, all concepts are more related to the exchange of goods than services. So if we wanted it to be exact we would also need a "Services" section (maybe even "Services wanted"/"Services offered", same way we got Buy/Sell).
What was the reason for separating Buy, Sell and Auction in the first place? I bet it's to make it even easier to find what you are actually looking for. If so, what is so wrong about my suggestion?