I would respectfully suggest to repeat the test with a faster shutter speed.
Why do you put the ISO to 100 in low lighting conditions? Put it to 800 or 1600.
Or turn on flash and take a shot at 1/250.
From the pictures you posted it is not possible to tell for sure that the camera has a problem (although it looks like AF is off), as there are too many possible influences. Even on a tripod with remore release (did you enable mirror lockup?)
As to the reply the seller sent you ... differences in sharpening settings of XT vs. 30D ... this is basically true.
But, IMO, it should not lead to such a big difference.
The other possibility, of course, is that it might have worked 100% for the seller with his lenses, but not with yours now. However, f/8 should give you a sharp image.
Best regards,
Andy
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