A quick Google search for Escrow services returns a number of them - as has been mentioned previously many are focused on commercial, investment or high-value transactions (e.g. house purchase). I did find one however that charges a flat fee of £2.99 per transaction for transfers up to £10,000, which presumably would be the sort of thing that would be useful for "our" sort of sales.
I'm deliberately not mentioning the name or posting a link because I HAVE NO CONNECTION TO THIS SERVICE, I'VE NEVER USED IT AND HAVE NOT VALIDATED IT PERSONALLY. It was more to point out that such services do exist. Obviously it would be important to confirm that any such operation is genuine, backed by / underwritten by reputable financial organisations, ombudsmen, etc before considering use.
One thing to consider is that although the buyer is protected from losing their money in such a transaction, I don't know if there's any protection for sellers who could be subject to shipping fraud, i.e. seller claiming the item never arrived and having their money returned to them by the escrow, while the seller loses their item and the money due.