I've seen similar, I tend to look after my gear extremely well, I use it sure, but all my kit's in pretty much mint condition (from my perspective).
I've bought second hand lenses where the previous owners state 9.5/10 in terms of condition yet when it's turned up on my doorstep I'd rate it at no more than 6-7/10 due to damage not shown in their pics, scratches on lense coatings, torn gaskets, creep etc...the sellers just seem to shrug it off, one said 'IMO it's 10/10 enjoy' & another said 'it wasn't like that when it left me, you must've damaged it!'
It appears to be that one persons grading is far removed from anothers...when I see 9-9.5/10 I expect very little sign of use & whilst not quite 'box fresh' pretty damned close to an 'as new' condition...others seem to grade along the lines of 10/10 for a lense that's 5 years old with a few dings & marks & say it's par for the course & therefore their kit deserves such a high grade...
I buy very little used these days having been burnt once too often - clearly it's a case of buyer beware in many cases.
I'm sure that not all sellers are unscrupulous, but I'm afraid, in my case at least, the few 'bad eggs' I've dealt with have left me with a bad taste so these days it takes a little longer to save for new, but I know if I have an issue I have a reputable store to go back to.