A rating system will be fine to implement.
Specially for some other trading systems require a proof of trust and to generate a trust system in the escrow.
The major problem with a rating system is when people will start using it without my escrow.
For example: if I see someone with 100% positive feedback with 90 transactions why I will pay fee to an escrow for very few risk of being scammed ?
I checked several possibilities but implementing the rating system while customers using my escrow seems not possible.
If you have ideas on howto make it possible you're welcome.