When I am selling, and a prospective purchaser wants to make a deposit to "hold the horse" pending a satisfactory PPE, I state in the agreement of sale that the deposit of $xxx has been taken, and the sale is subject to a satisfactory PPE, to be completed by a vet of the purchaser's choice, at his expense, within x days of the agreement. If the purchaser is not satisfied with the PPE within that time frame, the contract is null and void and the deposit will be returned forthwith. I also allow a PPE without deposit, providing an answer is given within x days (usually a week to 10 days), after which time the horse is subject to purchase by another party, unless a commitment and purchase funds are received before the deadline..
This works for everyone. I don't require a deposit, but the horse does not come off the market without one!