Happens ALL the time. Come to the lot, at first act like they might be a buyer, then try to pick your brain because they have a tractor in need of repair. Or call and do the same thing. I always say (and it is very true) "you might as well be talking to your sister or mother...I pay good people to do mechanical work because I am a mechanical moron. However, I have a very skilled mechanic who can go over your tractor at shop rates".
Worst I can think of is one fellow who brought his tractor in on a trailer wanting an assessment for trade in (or so he says). I tell him the mechanic is not here now, leave it, we'll go over it, and I'll call you (he had not called ahead of time). He made it clear he wanted a blow by blow of the trade in figure (in fact, he insisted on talking to the mechanic as to "why" it was worth a certain amount)...and I told him what we would do is tell him how much he would have to add to it to buy any given unit on the lot. Turns out what he expected was a free diagnostic check up--he had NO interest in buying.
I found it interesting that once I was clear with him what we would and would not do, he seemed to feel no shame in admitting what he was trying to pull off. I even offered to have the mechanic go over the unit (at shop rates....$45/hour) since it was already here--BTW, he was having significant engine issues according to him---and he refused.
I have come to the conclusion that many of these people truly believe that any dealer will take advantage of customers at every opportunity, so they feel justified in trying to take advantage of a dealer. i.e., they feel they have a valid reason to be a jerk. The rest who do this are just jerks for no reason.
Mind you, I see this as different from the guy who comes by, buys $20 worth of filters he could have gotten at Walmart, and then asks a couple of operating questions because he bought the tractor privately and has nowhere else to go. I like him better than the guy who wants the same info for nothing on a busy Saturday when we have real shoppers. It's all in understanding (or NOT understanding) what it takes to make a business run, and that time AND information = money.