I've not had a problem with this but I do make a point of having a fairly long telephone conversation when they first make contact and say they're welcome to take a look at the flock/herd with no obligation to buy. Once they're on the farm I get a good opportunity to sound them out and have been known, in the case of someone new to keeping that class of stock, for example, to very politely suggest they need to do some reading, go on a course, get their land properly fenced, or whatever else I consider necessary to make sure my stock is going to have (almost) as good a life at its new home.