So, I'm trying to access barter with my traders but for some reason it's defaulting to the the trade items mode. Is there a way to force barter?

Make sure that they are standing behind their counter. They will often have their hands placed on the shop countertop. Make sure that it's between 9:00 am (technically 8:00, but give them time) and 8:00 pm.

Find a chair nearby and wait in one-hour increments if needed.

Thanks