Let me say this from what I've seen browsing around ads.
If you got a 4X4, a couple years old, preferably Honda, mid-sized engine, you can get rid of that in a heart beat. I've noticed the same with YFZ's. If you're looking to trade down to a Z or something, and pick up some cash in the meantime, easily done.
However, as soon as you replace that stock muffler or do anything remotly related to the engine, people will back off.
I would suggest to only mod the suspension, and wheels and tires, and keep the bike spotless, it'll keep it's value well.
On the other end of the spectrum, you've got modded out, 6 year old blasters, which arguably loose the most value of anything I've seen. I'm trying to sell my bike and it ain't happening, I don't have the title, I've modded the motor, the bike isn't spotless, etc.
So in the end, I wish I was trying to sell a Honda Rancher 4X4, or that I hadn't put $1 into the blaster.
(It was alot more fun with it being twice as fast as stock though!)
Oh and another thing, with each passing year, you could loose anywhere from 5% to 15% percent of the bikes value, depending on age. So if you're planning on selling in the future, do it sooner rather than later.
Anyone else had similar, opposite experiance? Or any input?