my guess is that 48hrs of use would not be enough to cause any permanent damage. if you saw IR and it's been used for 400hrs, then I'd worry that it'd be permanent.
if you really want to be cautious, why not ask them to run the slides or a screen wipe for a couple hours and see if it clears up? I've never had IR that didn't get cleared after just a few minutes of that, so I'd think a couple hours would let you know for sure.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One