***UPDATE*** maybe someone will find this usefull - I only sold 1 dupe and drove out like 8 more out of garage and checked with iFruit if they are dirty ( and they were ). I have cleaned up ALL of my garages by replacing dirty dupes with new free elegies, I have checked every single garage for possible forgoten old dupes with iFruit ( if the total car selection number decreased I replaced that vehicle ). My newly purchased issy that i have upgraded was also flagged for some reason and like most of my arena cars so now im sticking with retro customs untill I am sure I can sell arena cars. After waiting 30hours of 1 daily sell limit I'm now able to sell atleast 2 cars a day. Don't ever sell dupes that you haven't checked trough iFruit!