You have heard it all from everyone else. Con men exist because people would like to think that they can get something for a lot less than it should be sold for. This does not mean that there are not deals to be found but no RV or TT should be purchased without a personal inspection and if you are not knowledgeable about the systems you are looking at, then also an inspection by a professional RV service tech - that you select and hire.
When you find something that you like, have spoken with the seller on the phone, gotten photos, a VIN # that you will have checked out, and are ready to make the trip, you narrow your choices down and then go see, maybe buy, and if you buy, take your RV/TT home yourself.
There are legitimate listings to find a pre-owned unit. Often these listings are connected with an RV/TT organization or club - like Good Sam, FMCA, FMCA sub-chapters, etc. If you look at local Craigs Lists you may come up with a bargain that you will go to see for yourself. Do not go long distance with a Craigs List ad unless you are willing to travel long distances and come back empty handed and disappointed. You might also check out local dealers lots. And there are legit dealers that you may have to travel to and get a good deal.
Lots of luck. Do all the research up front before you buy and will will be much happier later on.