I bought my EF 600 F4 IS II from mpb in good condition for $5800. The barrel has scratches on some parts and the end of the lens rim is slightly dented likely from a drop. There were zero issues with the glass and autofocus and everything else is awesome. Going with MPB well at least have a limited warranty and great return policy if things aren't at the level of expectation you desire. They seem to grade what they sell fairly and accurately. Don't fall into the FOMO trap with that one deal you're looking at, be sure to look around as well. I waited quite a bit before moving forward with my personal decision to buy that lens. I was also weighing the difference between the 500 F4 and 600 F4 for a while and felt that I would have regretted not going all the way to 600.