
My list:
10) Bodies that have windows that are supposed to be clear painted black (or any other solid color).
9) Cars less than 10 years old caked in a layer of dust so thick, the car must have done some time-tavel to acquire it.

7) Cars where the diffs are filled with some crazy choice of grease or oil - I've had 1:1 scale motor oil, a ball diff filled entirely with graphite grease, thin oil that dripped out of the gearbox, and shock oil.
6) Cars that have most oil and grease applied on the outside of the gearboxes

5) Cars advertised as "A good runner" with incorrect assembly that means it could never have run well - sometimes competition cars, which means the previous cars of the previous owner should have been worse...

4) Cars advertised as "A good runner" that looked okay in the pictures that turn out to have some 'original' assembly that both make them a bad runner and means you can throw various parts in the bin.
3) Cars advertised as "A good runner" that looked okay in the pictures and when you receive it, until you mindlessly try to give it a spin and discover the whole drivetrain makes a horrible stone grinding noise because of some 'original' engineering deep inside that makes you wonder what the owner was smoking.
2) Cars with crude repairs: wood screws to hold together broken chassis, nails as hinge pins, various unrecognisable bits to make the diff run 'smooth' (read: loose as heck, no diff action whatsoever), etc.
1) Cars with smartly hidden damage that was hidden from view on purpose, and when you point it out to the seller he gets angry and says you are the scammer, eg. gearbox that turns to have zero intact screwholes because they are all cracked and painted and sanded hot glue was used to 'repair' them.
Or maybe I should cut down on buying really cheap cars....
