I do not purchase with the intent to mod then resell. That would just be shooting myself in the foot given that part of what I offer at my business is CFW installs. For 3ds devices I charge cost of SD card (if the customer's is too small) + $40 labor. I'd much rather them purchase a fine used game console at the price I've set then inquire about that CFW sign.
With that said, If it is one of my previous consoles or a console I bought off someone and modded on a whim, I generally won't revert a console prior to sell. I will, however, remove things like freeshop/injected games if they are installed.