The old man trades four items always:

First: a weapon or an important piece of equipment. It's unique and it changes every month.
Second: a rare piece of equipment, not usually a weapon, that you can adquire only this way, but you can do it multilpe times. Changes every month.
Third: Always a chest key. The fish/es he wants for one of these changes every month.
Forth: Always baitx3. The fish/es he wants in exchange of bait changes every month. It's easier to obtain bait if you talk with the woman with a dog that's standing outside dojima's residence. You obtain always one by prssing the first option and she always appears on not-rainy days.