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Dealers will trade your car, and try to sell it as quickly as possible to make a profit. with modified cars that narrows down the buyer group and makes it harder for them to sell it which means it takes longer for them to see the profit. As a result they will lowball you an offer to 1) make you sell it yourself, or 2) make you sell it for so low that they can sell it for lower and get their money back faster. Its all BS i know but i think thats how most of them operate.
Also dont forget when someone comes to buy a car they will see a modified car and think boy racer, probably it was raced at every light. This you can thank for fast and furios movies