Vintage Mac sales are seasonal. I wouldn't try to sell anything of value in the northern hemisphere summer (ie now), and that would include an SE/30 ethernet card if I had a spare one.

MacMan's advice was wise: they do turn up on eBay in the EU from time to time, but mainly in the autumn/winter. At least one UK enthusiast had a small stock a couple of years ago, but he isn't actively selling at the moment.