Well the way i do it is see how much it will cost to build it now with the exact same parts and then go from there,now according to pc part picker your pc would cost $1800 to build it today.
Now you won't be able to sell it for that,now i don't know how you plan to sell it but if it was me and i was selling it on craigslist i would start at $1500,and if no one bites,lower the price and try agian.
i was accounting for that,it's just if i was selling it i would start really high to try and get as much as possible,but be willing to go down in price.
I remember i once a really old pc that was like a 2.2ghz single core 512mb ram,and a 32mb video card,i thought it was only going to sell for around $60-$80,but i posted it for $125 to start,and it sold for that.
If he does not have the good luck i did,i could see it selling for at least $800 to $1300.
it could be worth keeping 2 of the hdd's and either keeping them, or selling them seperately. Unless you have a use for the SSD i'd sell it with the rest as it fits in pretty well. i just think 3HDD's dont add much more value than 1.