how about each sale is done in "cloakroom" style.
each item has the sellers username, price (min offer accepted aswell perhaps) etc all on a tag on the item and the seller also gets a receipt. so for example
item number 232
seller rat
price £15
min £13
then when i come back later in the day i hand over my ticket with number 232 on it and claim the money paid up buy the buyer.
seller claims responsibilty to buyer should it be needed. ( shouldn't as buyer is paying with cold hard cash on the spot, after seeing the item in the flesh, unlike buying through the forum)
also another location I would recommend is the NAC stoneleigh warwick if your looking for somewhere still, though may be too expensive for this.