Klenger, I have found the two best places to find used trailers is the local newspaper and the various Auto Trader magazines. The Truck Trader always has some used ones, but the Heavy Machinery Trader has the most ads, although the majority are for new trailers. They probably have a regional Pickup Trader for your area, whereas the Heavy Equipment/Machinery Trader has nation wide advertisements.
It seems to me that when I need a used trailer of a certain type, there are not any advertised, then after I finally find one, I see several like what I want.
Yes, it is fairly easy to re-sell a trailer so long as it is not a specialty. The last one I sold after owning it for 3 or 4 years was slow selling primarily because it was a triple axle and very few want a triple axle. Also, it was only 6' high inside which is too low to get most cars in. The new one I mentioned earlier that I sold was a Haulmark with 7' inside height, dual axle car hauler. I sold it after parking it next to a busy street for one week.
Also, most people looking for a used one want a bargain. It is easier to sell it at a bargain price if you buy it at a bargain price. Just be sure the axles and frame are not bent, and that it is in solid shape. From what I have learned, Aluminum (side) is more preferred than steel because it is lighter weight and won't rust, but an aluminum trailer costs more.