What 3D Models do people buy?

Stuff that you can use in games, renders, animations and stuff like that.

To add to what @lockiabel said, you should also focus on models that are too difficult for the average 3D artist to do in a timely manner. Its about bringing value to the artist who buys your models. So If it would take me 30hours to model a rattan/wicker chair with intricate details and I charge my rate at $50 per hour that chair would take $1,500 of my time, so if I could buy it instead for $50 that is a very good deal.

Game-ready, low-poly, PBR. Stick to a style/ genre and you will be able to knock them out much faster than one-off, out of the blue models. Time is money.

That's also a good point by @SimonTGriffiths.

There's some good little money makers in simple things too. I remember seeing one artist making lots of 3D icons (like arrows, circles, particular shapes etc) and they can be sold as a pack for a reasonable price and takes much less time to make. If one of those packs is a hit you know you can expand on this area and build more like that or in that style.