It makes me me laugh that anyone who does not intend to buy outside the auction and asks if the seller will take another" buy it now" offer seems offended that you dare somehow "cheat" others who may want to bid. If they want the item so much then "buy it now' ...
It all depends on how the question is asked. I had an item for sale "BIN" and a guy politely asked if he could pay less, so I changed to "make offer", which he did and sold it.
Then again I have had some that were sort of rude and those got blocked.
Of course, YMMV.