Barton is an independent caravan merchant, and a main supplier for Caesar's Legion. He prefers to trade in Legion territories versus NCR territories, saying that he finds it cheaper and safer. This is due to Caesar not demanding (or needing) heavy taxes from traders in his territory and the Legion's effectiveness in keeping the trade routes secure from both raiders and creatures roaming the wastes. When asked about Legate Lanius, he mentions Lanius killing one of his brahmin just because it got in his way, though he goes on to say that he had enough sense to keep quiet and not seek reimbursement from Caesar.
If the Courier is on bad terms with the Legion, Dale, instead of talking and just saying random quotes, will say quotes from typical wanderers, even saying, "There's been reports of Legion raiding parties.", or mentioning Motor-Runner was put down. He will not offer the option to trade.
If Caesar has been killed, he'll ironically say that it "should be good for trade."
"Between having to hire protection and getting slapped with taxes, it's more profitable to stick to Arizona and New Mexico. But, I do cross the river from time to time when an opportunity comes along."