Value of the car

It depends on what you customize. If someone doesn't like the hood scoop, pins and rear wing, they can swap the hood and trunk for relatively cheap.

If you put in new car buckets, cut the dash, tub the rear, floor B&M shifter, custom door panels and all that, then it hurts the price and potential market for buyers because now people have to spend a lot of time and money to get things back closer to stock.

A lot of guys don't like the hokey look of aftermarket parts and pieces everywhere. A lot of guys prefer it that way because it looks more "muscley" to them. To each his own but as with anything, you gotta keep in mind you probably wont keep it for your entire life so you don't want to dig yourself into too deep of a hole.