If you buy second hand, I always recommend you get an invoice/receipt off the owner of the PC/Components. Imagine you buy that PC off him, and the GPU breaks tomorrow. You would have no warranty and he has no obligation after the purchase of giving you a receipt to get that warranty. People often get funny about giving away personal details such as an address when giving an invoice, but the way I look at it is that when you pay for a product, you're also paying for that aftercare (i.e. Lifetime Warranty on all DRAM's by Corsair).
With that out the way, for what he's expecting, I'd say $450-$500 would be fair. Otherwise, you might as well save up for a new PC.
As stated before, it would be nice to have RAM above 2133MHz with Ryzen...