It doesn't even slightly work...
My L85 was £320 new 2 years ago.
Externals have cost me something in the region of £500-700 + about £350 on mags and batteries/charger. So I've put £850
I've spent £250ish on the internals
I've factored in 5 months of heavy usage, since I've owned the gun for just under 2 years now, but have never treated it with what I would call "heavy usage"
Generosity factor I've left blank because I don't know what the shit it even means or what values it operates by.
So that's a gun that's cost me £1500ish and apparently it's worth £922.
If I put 22 months in the heavy usage section then it gives me a total sale value of £837, so 17 months added heavy use only accounts for a price drop of about £85?! Who wrote the maths coding behind this thing?!
I think I'd be lucky to get £600 for it. I'd probably put it up for sale at around £500. Internal mods rarely count for anything in a second hand sale, people would rather have the stock parts because then they can vouch for the parts not being badly chosen "upgrades" that are worse than the stock parts, the accessories lose a massive amount of their value, in fact I'd go as far to say that the value decreases exponentially the more accessories there are, so all my mags, batteries and external parts would probably only give me about £200 at the upper limit on top of the original price of the gun, which would drop to at least £250, and that drop is only as generous as it is because of all the parts it comes with.
No way is anyone going to buy a gun for £922 when they can get a brand new, stock one for under £300.