Did I overpay for my used RTX 3070?

It's kind of a lukewarm deal. The RTX 3070 is a solid card, but with newer models out, you're not exactly getting a steal here. If you're looking for more power efficiency and performance, the 5060 is only about $50 more and would probably be a better investment.

Honestly, you probably should have checked the prices before buying. There are listings on eBay for around $240-$250, so your price is pretty much the standard. You could've gotten a better deal if you'd done a bit more research.

For $250, it's not terrible, but I personally would have saved a bit more for a better card like the 9060 XT 16GB. The RTX 3070 has just 8GB of VRAM, which is its main downside. If you're okay with it, then it's a decent buy, but keep in mind some alternatives might serve you better.