Definetly go for a model with at least one slideout if you can - it makes a huge difference.
If you are looking for used, you will get a much better price in a private sale. But, you are getting it as-is. No dealer to go back on if you discover issues after the sale. So, make sure you look it over good and test everything first. You'll be able to tell pretty quick if the owner took good care of it or not. If its dirty (inside or outside) streaked, faded that's a red flag. If the owner was too lazy to wash it, what else didn't they do? Like keep the roof sealed, lube the axles/bearings, winterize it properly? Travel trailers aren't like houses - fixer-upers are usually a bad investment.
You should get a sense pretty quick of whether a trailer looks like a good possibility, then go through it very thoroughly.
And don't discount your bargaining power with cash. Selling a cheap popup on CL is easy. People snap them up. Selling a more expensive tt in a private sale is tough. The number of buyers is limited, so if you do find the perfect rig, you should have a good position to negoitate from with $$ in hand.
2005 F-150 FX-4
2013 Rockwood Roo 23IKSS