Question: At Los Santos Customs, I’ll receive an error saying "Sorry, you can only sell vehicles worth up to $50,000" when attempting to sell certain cars. The "sell" option is sometimes missing as well. Which vehicles cannot be sold to Los Santos Customs?
Answer: You can sell Personal Vehicles, vehicles stolen from NPCs with value under $50,000, and any vehicle purchased from the following in-game sites:
legendarymotorsport.net
southernsanandreassuperautos.com
warstock-cache-and-carry.com, as long as the vehicle can be stored in your garage.
You cannot sell vehicles that are obtained from Warstock Cache and Carry which require Pegasus for delivery, nor can you sell vehicles obtained through glitches or exploits.