looking to start soloAds.org

.. > with the idea that people could buy and sell
.. > solo ads there. Would the best platform be a forum?

I ran such as system many years ago. Worth noting, most every list owner reports inflated list size. Plus, many have deliverability rates under 50%. (some even lower)

This is typical:

Advertisers pays for solo ad to 20k list.
However the ezine owner has misrepresented his list size as being 20k list, when in actuality it is only 12k, but after blockage/deliverability issues only reaches 5k subscribers. What happens next:

1. Advertiser has lame adcopy & subjectline.
2. Advertiser pays for and expects their ad to reach 20k subscribers.
3. Advertiser's solo reaches only 5k subscribers (1/4 of what was advertised) due to blockage/deliverability issues.
4. Advertise complains to ezine owner, saying his ad didn't pull well.
5. Ezine owner claims that the fault was with the adcopy.
6. In some cases Ezine owner offers to run the ad again (for free) but second time around still has poor results for advertiser.
7. At this point some advertisers give up and walk away, others will file a Paypal dispute.

My advice would be to totally remove yourself from being responsible for the transaction & performance between the ezine & advertiser. Position yourself like newspapers and craigslist do for classified ads, where they're not responsible for anything tha takes place between buyer and seller. Even when you do that, expect some advertisers to keep bugging you (owner of the ezine directory website) claiming that you need to ban ezine owner xyz because they're a scammer. (dangerous waters, you don't want to get dragged into disputes between ezine owners and advertisers, yet you don't want to be a place where scammers can advertise and take advantage of people)

Another bit of advice: Stay away from ezine ad co-ops. (advertiser pays to have their ad run in say 12 ezines) About half of the ezine owners in the group don't run the ad. They figure that because the other ezines in the group will run it, the advertiser won't notice.