And is there a way to determine what a entity might bring to trade?
They bring whatever they're allowed to 'produce' offscreen.
They will bring all weapons, armors, clothes, instruments and toys you have packed in their entity.
For vanilla df:
[USE_GOOD_WOOD] tag determines if they will bring Feather Tree logs (EVIL for Glumprong).
[USE_GOOD_PLANTS] is for Sunberries (EVIL is for Sliver barb).
[USE_MISC_PROCESSED_WOOD_PRODUCTS] Charcoal, ash, lye and potash. Haven't seen anyone bring this 'randomly', so propably it determines if you can ask for that stuff via entity guild representative.
[RIVER_PRODUCTS] Will bring you river fish. [OCEAN_PRODUCTS] - ocean fish.
[SUBTERRANEAN_CLOTHING] Will bring silk and gcsilk thread/cloth (and finished clothes with conjunction with proper clothing tags). Also, with [USE_CAVE_ANIMALS], various leather clothes (giant bat leather, cave crocodile leather and so on).
[CLOTHING] Rope reed thread/cloth (and finished clothes with conjunction with proper clothing tags).
[ANIMAL_PRODUCTS] Meat, leather, milk and cheese.
[OUTDOOR_FARMING] They bring aboveground plants (Fisher berries, longland grass etc.)
[INDOOR_FARMING] They bring undergrounds plants (dwarven plants)
Also, for example, they must have [PERMITTED_JOB:CLOTHIER] if you want clothes, [PERMITTED_JOB:CHEESE_MAKER] for cheese and so on.
People often just copy all job permissions and the community screams 'oh modded races just love bringing you cheese'