Dave,
On the main p.c forum that i use they only have computer related items and it works just fine, i'll run with whatever.
Here's a sample of some of the rules they have.
1, A price must be posted with everything (no "offers")
2, All deals are to be done on the forum (PM's are deactivated) Reason for this is so that there is an upfront and open "pecking" order for stuff and no backdoor deals via PM, also members get an idea of current prices for components.
I'd prefer a price to be posted with stuff rather than "offers please" - it's just too vague.
The seller has an idea what he/she wants for the stuff they are selling so why not, if it's too high then after 24 hours allow a price drop.
Just my 2p