Look on cars.com or autotrader.com, and every vehicle listed has the VIN published for all the world to see. If there was something people could do with a VIN scam-wise, then you wouldn't think anyone would be willing to list one.
I don't blame a potential buyer for asking for a VIN. The VIN on an automobile can be used to check records through services like CarFax (ie., has it been in an accident, does it have a salvage title, etc.) Perhaps the same information is available for trailers as well. Additionally, the only way you know for sure what you are bying is by decoding the VIN. I have seen plenty of craigslist ads where the seller doesn't know what model it is, has the year (and even the decade) completely wrong, etc..