There are goods in some systems/Houses which are produced for a domestic market.
The prime example is MOX, produced on LD-14 in Bretonia. Other Bretonian bases and battleships should be buying it, since it's the fuel they're supposed to use.
However, the only way to profit here is to deliver to Liberty, which seems to make it look like either A) Bretonia doesn't need fuel or B) Bretonia gets its fuel from Liberty and vice-versa, despite having a large domestic source of MOX. It would, of course, make sense to use domestically produced fuel, right?
Maybe I'm missing something, and I do understand the need to make traders travel a bit... but... it doesn't ring true.