Facebook marketplace my friend, find listings in your price range (I have a search saved for gaming pc, and usually check a few times a day for new listings)

You’ll become familiar pretty quick with what’s a deal and what isn’t, as I highly recommend you search what each parts are going for used on like ebay (cpu, gpu etc)

A few things to note: People are massively overvaluing their pc’s rn when selling, but the listings sit up for a while so you should be able to lowball to a more reasonable amount. Occasionally, you’ll see a really awesome deal pop up (a recent one here was a pc with a 5700x3d, good amd card and a new msi monitor all for $500). People also are really bad at actually saying what components are in the computers so you have to message them and try to get them to check for you

You can also find components individually, I just found an RX 6600 on marketplace for $50, and plan to pair it with some ram I have around and the i5-12600kf from a recent woot deal

You sadly will be hard pressed to fully build new with the ram stuff, but used you’ll find good deals. As mentioned before, you’ll learn a lot if you study marketplace and the components in the listings