Population density close to the border is way too low to sustain such a thing, IMHO. Take a look at this image. The lighter color means less than 5 people per squared kilometer. It's basically just jungle over there.

So, many people crossing over is just not going to happen. And a few people working as intermediaries is dangerous (the intermediary may run away with your cash, or call too much attention to himself and end up being caught). I know, drug dealers do this all the time, but well, they're drug dealers. They may charge a huge price for their services and still sell their product. I don't think the same applies for cash conversion.