I break Z3's so I can tell you what sells, and what is just worthless.
Good mirrors sell for £70 upwards if uncorroded.
Sills always sell if decent condition - £75 a side gets them gone.
Throw the rear lights in the bin. You might get a tenner each, one day.
I leave bonnets on the shell when it goes to the crusher. £40 is a very fair offer and probably the only one you'll get. I've never sold one.
Ditto boot lids and rear bumpers. Just take them to a scrapyard. I've been selling a spare wheel cage for over year.
Good pre facelift rear wings should sell as they're rare I'd aim for £75 each if they're really good. Any rust, just bin them. Later ones are £50 each, loads around and no buyers.
Z3 parts are a very hard sell which is why I don't really do it now. The cars aren't worth much and there are mountains of bits on ebay as so many have been chopped up.
Take the first offer to be rid of it!