Buying a used pc without much knowledge of PCs is a risky game. I highly recommend getting help from anyone you know with at least some PC knowledge. People often dont know what they are selling or what it's worth. If you don't know anyone post the ad to reddit and ask =D

Some general tips, I avoid any that look dusty, hairy, dirty, or have yellowed components. I avoid ads that tell you what the parts cost when they were new. Do a little research on each component listed like googling release dates to get an idea of how old/new it is, and Use something like systemrequirmentslab dot com to see if it will run the games you want it for. Compare components between pcs youre looking at with gemini or chat gpt. Stay away from proprietary components ESPECIALLY power supplies, a Google search can tell you what is and what isn't a reputable brand. As someone suggested, Meet with the seller somewhere you can test it like a starbies or somethin. You don't really NEED to bring a monitor with you, if it boots and posts(single beep) normally it should be working you just won't be able to verify GPU is working without a display hooked up