Haven't given it much thought but here are a couple of ideas:

1) Users pay to list their domains.

  Pro: Guaranteed revenue from every user
  Con: Low motivation for user to pay up as there's no guarantee for reward

2) List up to 20 domains for free anything over that falls into a tiered annual payment plan

  Pro: same as 1 but better because of larger free user base who might be more likely to convert to a paying user
  Con: Could a user just keep rotating hundreds of domains through their free account? (imposing a limit on how many times a user can list and de-list domains would prevent this)

3) Users pay a flat fee only when a successful trade is made

  Pro: more motivation for users to pay up as the reward is imminent. Also both users can split the fee making it more affordable for each of them
  Con: Revenue is conditional on successful trades. Could the users use the service to simply list their domains then use outside channels to complete the actual trade/transaction?

4) User pays a fee when they want to take a domain they received through trading off of the trading block. This would require the site to actually assume ownership of all domains being traded then hold them hostage for a small fee when a trader wants to take possession of them (or else they are still up for trade/in limbo)

5) Maybe the site handles trades as well as cash purchases. Trades are free.. purchases incur a 2% fee.

6) Ads for other internet services. Perhaps the domains could be contextually paired with the ads so that if a user is searching for mma-website.com he/she sees ads for mma training equipment and facilities.

7) Make the users redirect the domain to you and put a nice notice on it stating that the name is available for trade / swap / a worthy purpose and place some AdWords on it.

You can actually sell it as a value-add and, from what I understand from professional domain squatters / traders, there's a surprisingly decent amount of money in it. The amount of link juice you could give the main domain probably also isn't insignificant...

There are plenty of other services you can easily sell/advertise to an audience of people who are swapping domain names.

5$ a pop?