are trade feedbacks mandatory?

No not mandatory, up to you if you keep it or not.

you should leave +rep but note that the trader doesn't leave +rep in return, and change it if they do later

It's never mandatory in any way.
However: one could consider it a sign of a polite trader.
It's totally your choice whether to delete the +rep or not.

Feedbacks are not mandatory, but if the other party doesn't reciprocate I'd just remind him politely later after several days, and remove if he doesn't. It's the simplest approach.

No, you aren't obligated to leave feedback and he is also not obligated to you give you feedback. But it's polite to do so.

Thanks Everyone :)