The users might cheat you out of your commission. The buyer and seller have an incentive to agree to sign a transaction that sends all the money to the seller, nothing to your site.

However all transactions are public on the blockchain so you could see them doing this and take action, such as banning them.

It's unlikely because I guess your commission will be small (bitmit had 1%) and people would generally recognise how useful your site it and won't be so nasty. I know there is a drug marketplace called The Marketplace which uses 2-of-3 multisig. I wonder what they've done if anything against buyer and seller collusion.

Or you could have a model where commission is only collected if there is a dispute, or where the seller pays a small fee for the advert, or where buyer and seller have entry fees, etc etc.