'03 Nissan Frontier cat - what's it worth?

Guy called me earlier today. Asked if I buy cats. I said I do, but that I am still new and would have to check on some things and get back to him.

Says there are 2 cats he replaced. Does the Frontier have 2?

Reason for replacement is failed emissions. Truck has 180,000 miles. I asked if the honeycomb looked complete. He said he doesn't know, but that it hardly rattles. (I assume this is a good sign?)

I couldn't think of anything else I should have asked. Don't wanna get burned. This will be my first cat purchase. The rest I have came on vehicles so I had room for a profit.