The do-it-yourself PC market is really crazy right now. If you are on a $400-$600 budget you'll have a hard time building a decent rig, unless you really scrounge for deals on used parts.
You should have no problem buying a motherboard, case, power supply, SSD and RAM new. Watch slickdeals.net or reddi*****m/r/buildapcsales for deals and jump on them when you see something you want. A low-end B450 motherboard would be fine for a budget rig. I paid $68 for an ASRock mATX B450M PRO4 motherboard from Newegg a few weeks ago.
With your budget, I would try to find a used Ryzen 5 1600 AF, 2600, or 3600. eBay prices are insane, so I would stick to forum marketplaces or the hardwareswap reddit to find the best deals.
Graphics cards are the same way. Demand is so insane even the low-end cards are out of stock or overpriced at most retailers. I would try to find a GTX 1060 6GB or an RX 580 8GB for around $120-$140 shipped. Most people are trying to sell these cards for $200, but they are not worth that.
Obviously I am US-based. If you aren't, I can't really speak to the secondary markets in your country.