>>1806638100 bucks is a good number
look for brands that have a decent name to them like fuji, trek, giant, specialized, cannondale, ect. avoid walmart brands or big box store brands.
look for rust , make sure shit isnt loose or wobbling when you check out the bike, give it a test ride and go through all the gears and brakes, check the seatpost isnt super loose or stuck. Check how true the wheels are and pray it doesnt have a bent axle. Idk iv gotten some really really amazing used bikes for great prices but also have gotten some lemons iv lost money on, the more you do it the better of an eye you get for looking out for what you want.
anyways good luck anon, and dont buy used carbon, if you are buying aluminum check for dents, if you are buying steel check for bends or rust.