Cowboy
You'd be charged with handling stolen property.
Lets say, you buy a BMW from a man you don't know that has a book price of £10,000, for £2000.
Obviously, most reasonable people on the Clapham Omnibus would assume that there was something a little bit wrong and not buy it without some very serious questions and enquires.
So if you can show you were offered a bargain that was too good to be true, but did everything you could to establish that it was legit, then you'd be fine. If you didn't then you are likely to get charged.
In this case, £300 for a caravan seems to me to be a tad suspect, but admittedly I've never bought one.
However, obviously the CPS have decided there's not enough evidence of a criminal offence, so once thats sorted, if someones actually using the property as a home, it is very difficult to justify simply evicting them.
A Court is the only place in the circumstances that can be decided.
I don't agree with it, but I don't make law, nor do the Police, there's 650 odd bent gits in Westminster that do that.