lets play devil's advocate if your selling this to someone that can read the labels of hardware
and how I see used Systems
1. considering you can buy a G4560 that performs the same as an I3-6100 for 55$ NEW might I add, your CPU isn't worth more than 30$
2. Motherboard sells on Newegg for 56$ right now New, so I see yours as used for 30$
3. single stick of ram, how though I hate you so..... but new 55$ on amazon, so because I dislike having to get another set of ram ... worth 0$
4. powercolor r9 270 TurboDuo 2gig, beast of a video card, assumed non overclocked, an easy 120$ Used market value.
5. LCpower LC6600 V2.2 , low end PSU, worth 40$ new, used worth nothing as it will have to be replaced. Value 0$
6. monitor lg 25UM55 21:9, nice, 2K monitor, ebay last 10 sales of this type of monitor averages about 140$, so I would accept that value
so basically 30+30+0+120+0+140 = 320$ would be the max I would pay for such a system