Interesting question.
I've never heard of anyone getting a discount for a display bag unless there was actual evidence of wear, I'd certainly ask and prefer if they had another. For the money 'we' usually spend on bags, a discount of about 10% is not going to make me buy a defective bag because I could buy a near perfect bag pre-loved or in a sale for much less.
If I notice anything amiss with a bag, there would be no way, not even for discount, not if it were the last, unless we are talking what would happen naturally in 5 mins anyway. Even the odd scratch is not acceptable on a really expensive exotic bag, this has happened to me twice on bags that I was head over heels with. I've rather waited and ordered another, this is one reason why it's sometimes best to go at the beginning of the season.
Not buying the bag on display can work against you though. SAs will often put out the best of a batch, the best 'pattern' of python, the best lined-up pattern, a print where the picture is the most centred etc. If I examine a display piece and it's perfect I'd rather buy that than a less-than piece from the back. That happened to me with a croc wallet. The display piece had even, symmetrical scales back and front whereas the 2 from the back had smaller scales on the front than the back and the display piece had been under lock and key in a glass display cabinet for 2 days.