What improvement to the forum does one make by advertising Ebay Auctions? It implies that you're not really selling it here but somewhere else and that you have to have a bidding war there in order to get it.
The alternative to advertising your ebay auctions, is to have a set price in mind and offer it here so that you don't have to wait 3-7 days to sell it and you don't have to wait up to that amount of time till a seller is ready to sell it to you.
I guess I've been spoiled by a PC hardware forum that actually does buy/sell and trade on the spot and doesn't advertise auctions.
Example:
http://forums.anandtech.com/categories.cfm?catid=45
A good reason for ebay auctions is that it could increase posting activity(but may decrease pure buy/sell value). If the assumption is that it's difficult to auction off mac hardware on ebay, then free advertisement in message forums isn't such a bad idea.
But debate between the ability to negotiate prices and get to the transaction on the spot as opposed to Ebay.
Sorry for the rant. I'm more of an impulse buyer/negoiator than an ebay buyer and I think this place could be more fun if we had more serious transactions here(just be sure to have the full contact info of the person you deal).
How about a prefix system for posts in this forum:
FS = For Sale
FT = For trade
WTB = Want to Buy
OT = Off Topic
Ebay = Ebay Auction
AD = Advertisement(as in "MacWorld magazine subscription $19.97!")