You have to consider who is going to collect the money and how that process will work.  And are you going to be the complaint department when things go wrong.  Money changing hands between three partys can be a train wreck. 

You also have to consider the fact that in order to do this it will prob be much like the financial plugin pocess now. You cant just buy an all inclusive plugin for finance that does it all.  You need multiple plugins and they are all on average of $20  

Is a site owner going to spend $60 in order to run an auction, i don't know.   I myself was going to get the classified plugin but then i discovered you need the other plugins to go with it.  So then i started thinking maybe it is an expense i can do without right now.  Right now i do the for sale and garage sales thru the forum and i am not in the loop when it comes to payment at all.

So i ask my members how they felt, would they use a classified page or is the forum good enough.  The result was about 70/30 to keep it on the forum.  

I do like the idea of a auction plugin but it would have to be all inclusive and very affordable.