I have sphinx search in the backend but turned it off because the number of posts is limited.

Thanks for the feedback, I'll look into making browsing/searching across regions better.

Do you think it would be useful if you are visiting a category in your local city to see posts from other nearby cities (within 150 miles)?

Oh yes, most definitely. I was recently in the market for a new (to me) dirt bike and it was a pain switching around to different local cities and performing searches. And, because I was willing to drive for 2-3 hours to pick it up, some people would say they lived near a larger city to get more hits but when I called them up it turned out they lived like 50+ miles away from said city.

How about arbitrary distance, which the user defines?