Looks like an M103/105 series trailer. with flatbed I usually saw them used for generators, but could also be cargo.
Whether it's worth 500 depends on you. It's an air brake trailer (I can't see if this one has them, I'm not really up to speed on this series so don't know if the early ones had them or not like the M101/M116 series), so either have to tow it with an air brake vehicle or swap the axle to one with electric brakes.
They are usually pretty solid, but they are heavy. Very heavy for their size/capacity compared to civi trailers.
Unless you really need to hold a lot of weight on a short wheelbase and have it high off the ground (or want to lower the trailer) then they end up being the type of trailer you park and store stuff in/on until you really need it.