What is a "copy background"?

[DataInTheStone]

A lot of cels on buyee or mandarake are listed as having their "copy background". What does this mean?

[Gonzai]

Means it is not the original. It is a copy of the original. Since cels have so many layers, usually the A-1 cel keeps the original background, though, I have seen the End cel keep it sometimes as well. All other cels, if the owners take the time, will make copies of the background and put them with the cel when they sell them.

[tarakatsuki]

And even if the seller doesn't specify, it's easy to tell when you receive the cel. The copy background will always be a thin piece of paper, while the original is a much thicker paper with actual paint on it. And remember, original background does not mean it is actually a matching background.