a trusted dive shop and friend down there.

Before leaving, I took a dozen or so pictures of the condition of the van with the guy from behind the counter IN the pictures. The transaction went smoothly and I didn't have any issues at all.

The only attempt we had to swindle us was while gassing up at the station outside of one of the ruins. I had been warned, so I was looking for it. Firstly, I made sure that the pump was zeroed out prior to the guy pumping my gas. I hopped out of the van immediately before he could start pumping and checked that. After pumping, he told me it was 100 pesos or something and I handed him a 500 note, he very quickly tried to swap it for a 5 note in his overalls. He was fast, but I was watching closely. When he tried to tell me it was only a 5, I reached over and pulled the 500 out of his overalls. End of that story.

If you search some travel forums, there are some good recommendations and tips on who to avoid.