I can only see problems.
It's not a bad idea, but due to shipping, it seems much more of a localized thing. Granted the board could be setup based on locales. Still, I'm just not sure it would work so well.
By the time you factor in locations, plus how often people purge during transition, versus how many are in transition at the moment, it just seems like a needle in a haystack. Especially if you factor in sizes. It's a lot of details to match up amongst a small population.
In terms of legalities...
That isn't so difficult. Basically the person running the site absolves themselves of all responsibility and does not interfere unless legal action amongst two people occurs. You don't get involved and you don't take any responsibility. All transactions are a deal between the two people, the site simple facilitates contact.
Ebay gets involved because of how it runs. They take payment for the transactions, they rate the transactions, and they handle payment for the transaction. They even facilitate safe harbor and shipping. Therefore they are held somewhat accountable.
Limit yourself to only facilitating contact and legally you are safe. Yes, someone can bring about a lawsuit, but it will not get far (standing in a court room is a near impossibility). The most you really should get involved is a request for IP address, time/date and email address, from a court, lawyer or police.
As for the economics...
It would have hosting fees to deal with ($3-$10 a month, plus domain name, and I would recommend a private registration. For a com,net or org, that would be about $20 a year (.info is about $12 for private, but I recommend a com,net,org). The software could be had for free. So a low ballpark figure of $48 to $140 for everything.
Most ad companies will not deal with a low hit site, which this would be. Google used to. Almost none pay before you reach $100. It takes A LOT to reach $100 with banner ads. Figure 120,000 views before you hit $100. In other words, it won't support itself. Granted it's not a lot, and with some marketing you could easily get that with private ads or donations. However, it has very limited appeal and returns and the whole point is about being cheap. So I wouldn't count on donations too much. Trying to sell private banner ads is like pulling teeth.