When you buy a guitar, love it and start putting money into it, which I must say there is nothing wrong with doing, as I do it all the time. For example stuff like new pickups, maybe tuners, other hardware, bridge, etc. They can add up fast.
For this discussion I will refer to a Fender Mexican Stratocaster on the used market. Why? Everybody has played one, a ton of us own them, very common and easy to find in any market. I have owned over a dozen over the years, paying between $175-$275 a pop. They are quickly bought and an easy sell.
For this we will say we paid $300. Its a killer guitar, plays nice, but the pickups are a bit bland. that adds up to $150 really fast, no matter if you go Lace sensors, or Seymour Duncan, Dimarzio, etc.
What is the guitar worth? most certainly not $450.
You throw some locking tuners on it (which I hate and I promise I will do a blog on in the near future). lets say, $50.
What is the guitar worth? most certainly not $500.
Lets mix it up...
You find this beautiful American Strat that you are lusting over and want to buy it. However to afford it, you must sell the Mexican Strat, So you list it on Ebay/Reverb/Craigslist/Gumtree,etc. for $500 because it is 'heavily modded' and you have 'a lot of money invested' and wait. THAT IS YOUR MISTAKE.
Nobody in their right mind will pay more than $350 for that, even if it is perfect.
This is what you should have done: KEPT ALL OF THE ORIGINAL PARTS and swapped it back to the way you got it. Nobody is going to give you that astronomical figure for that guitar.
What you have is a guitar with the original parts in installed that your sold for $300. You also have three nice pickups that you could likely get $100 for (or to use on a different guitar OR your new American Strat), and a nice set of locking tuners, ~$30 or again future use.
You lose a LOT LESS MONEY $$$ this way. It is something that I have always done to any guitar i have modded. For the guitars I do have that were modded by me, I have filing cabinets FULL of parts. If I get rid of the guitar I open up the drawer it is in, swap it back, then photograph and list it.
**MOST used gear that is bought used smartly and in the same condition sells within +/-15% of what you paid. That is NOT a published number, just experience of buying and selling a lot of guitars and being glued to the community both on the net and online.**