I sell zx81s all thee time on eBay, but I charge around £100 for each one.
I bet that's a lot more than you paid for yours. Why? Because mine are all tested, modified for modern TVs, checked and guaranteed.
I'm not just plugging ny store, though. My point is that eBay isn't at fault: if you buy the cheapest you'll get the worst quality, spend more and you'll get higher quality. EBay isn't like a shop, it's a market full of shops, and the best deal is usually not the lowest price.