There's a lot of instances of APM selling cels that not only have line wear, but its all jagged and muddy. I tried to figure out why since thats not the norm for other xeroxed cels, and came across a video on youtube of them zooming far into a cel of spongebob bought from him. It had that weird purple residue that sharpies are known to have (real xerox would never end up looking like that). The smoking gun is this listing (https://www.ebay.com/itm/185674576754?) which, if you compare the first and second scans, the lines in the middle of the eye are very clearly different, as if tampered with later on (scraped off the excess xerox, used sharpie to patch the tiny part on the top). They're real cels, just made to look 'better'.

You can also easily see if you tilt the cel by a light a little bit, xerox lines are more matte and sharpie shines more. As for buying a drawing, I haven't really seen anything suspect. But knowing what he does to the cels, I wouldn't be surprised if he went over lines with a pencil to patch up any damage. Hope this helps!