Is a used RX 5700 XT worth $135?

That sounds like a reasonable price if the card is in good shape. It's a significant step up from the 1650 and should still be useful for 1080p gaming. I wouldn't call it an automatic buy without checking what else is available locally, though.

For me, the catch is the age and feature set. The 5700 XT doesn't have hardware ray tracing, so if that's something you want, keep shopping. It can use some software-based upscaling, but it lacks dedicated upscaling hardware too. At $135, that tradeoff might bother you more than the raw performance difference.

I'd ask to see it running before handing over the money. A game or GPU stress test can help reveal crashes, display artifacts or unusually noisy fans. Check temperatures during the test and inspect the card for damage too.

The triple-fan cooler is fairly big, so check your case clearance and the power connectors on your particular PSU. A support brace may be worth considering if the card sags once installed.

Depends on your resolution and games. For 1080p, the extra performance and VRAM over the 1650 are appealing. At higher resolutions, 8GB can limit texture settings in some newer games, so don't expect to max everything out.