My £0.02
There's no 1 answer, depends on the car, what mods, how far from standard and how quickly you want to sell.
I had a modified mk4 polo, which was very much done to my taste, and there wasn't much of a market for modified ones. I removed coilovers, wheels, VR6 leather seats etc before selling, made more money that way. It still had a couple of mods (smoothed, de badged, tints etc.)
On the other hand, I sold my mk4 golf gti with mods intact. In this case I believed the mods I had done appealed to more buyers (the car had good feedback on forums and at shows/meet ups etc.) and I sold it for over book value and fairly quickly too. It had upsolute chip, coilovers, dished 18" wheels, smoothed r32 front bumper, anni skirts, milltek, smoothed rear, tints, and a decent audio system (genesis amps, JL sub, uprated components, hidden false floor, dynamat sound deadening.) I may have got slightly more if I stripped everything, but not by much, I worked it out by checking ebay prices for the mods.
Hope that helps!
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