I bought a used 4870 a few months ago. The seller seemed pretty sketchy (sounds like he bought it used to try and print money). My spidey sense told me to walk away, but I'd driven an hour and a half, the price was right and the unit itself looked fine, although the box was beat up and the documenation was missing.
I've done about 20 scans, and while they don't look bad necessarily, I don't have any frame of reference to compare them too. I've looked around for a lab that has an Epson flatbed locally that I can use for comparison but nothing's turned up. I've gone through the process of shimming the holder using business cards and found that the shapest plane is with no shims.
Any tips for things to look for in the scans that would tip me off of any issues?