Private party used is where you can get the best value. Do some homework over a period of time on Craigslist, RV Trader, etc. so you know where whatever you are seeking should be in terms of price. People will frequently set the price at what they owe, which is often more than the trailer is worth. Nothing wrong with buying new and you can get good deals sometimes, but you will take a significant depreciation hit when you pull out of the dealership.
I agree with the comments above about water damage being the biggest thing to look out for.
Edit - budget in replacement mattresses. There are great foam options on Amazon for reasonable prices.