Trust of the community, experienced/knowledge of trading/deals/transaction to other users (p2p deals) are one of the keys to be eligible on escrow services. There's no required rank for it but many recommend for higher ranks as they spend lot of time in this forum.

Holding fund is not the only work of escrow, handling disputes is also one of the criticals you need to work on.

So to start on this service, you need to build your own reputation.