I had acidwently sold my barnfind audi but it thankfully didnt sell but it now shows up as not owned in my garage even if i own it.
Did you ‘reclaim’ the car from the AH ?
I did
If you check the known issues is says “Owned tag goes away on the barn find menu after trying to sell a barn find car.”
You will get another from upgrade heroes just in case you had sold it so it’s natural to try and sell one. I sold my second one first time and never had this issue. 
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At least it dosent do much