I wouldn't make an offer. The way the ad is wrote you can tell they're not seeking offers, but want what they ask. They think it's an awesome deal someone should jump on, so awesome that if folk are too stupid to see what an amazing deal it is, they'll fix it themselves and sell it for a whole bunch more money, or so they say...
You can see in the pics it is twisted at least 9" or more (check the alignment and count the deck boards on pic 6 of the ad)... The $500-700 mentioned above is all that it is worth in the condition it sets, tops. At best you end up with a pair of axles, suspension, the tongue, and some bits of hardware. The rest needs redone from scratch. If you could somehow pull that 9" worth of bend out of it, you're going to have to add hundreds of pounds of extra bracing to keep it square and support the areas that were bent or stressed by the bends. I'd personally chop it into pieces to use the straight parts as bracing for an entirely new trailer frame and recycle the rest.
By the way the ad is wrote I have a feeling they would take $500-700 as an insult and be a real pr1ck. I'd call and go look, but I'd refrain from making an offer, just watch for a month or so while he wakes up to the fact that is an insane amount of cash for twisted steel. When/if you go look, take a jack with you and raise it up and spin each of the tires to ensure they still rotate properly. Good chance with the impact that unit took, the axles are likely not that great anymore...