You can't really second-guess the reason why a seller uses the best offer function, it's not all about giving the buyer what the buyer thinks is a good discount on a single purchase.

Without any more information in your post to base an informed reply, I can only point out a couple of things..

Some sellers definitely do waste time with the Best Offer function, some also misunderstand its purpose and wait for offers higher than the BIN price

On the other hand, some sellers use it with very specific intentions, such as discounts for multiple item purchases, and they will not even entertain 50c off if someone is only buying one thing.

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I had to laugh at the bit about some sellers waiting for a better price.

You have to wonder why on earth they would think  a buyer would offer more than the price they can buy it now at.

As a buyer, my understanding of it, when i first saw it offered was that if I wanted it, I could pay the full price there and then with BIN.

That way I definitely would get it.

But I could make a 'best offer'. To me it was obvious the best offer would be lower than BIN and may or may not be accepted and someone else, in the meantime, could jump in and BIN or make a better offer.

At no time did I assume it meant I could only make an offer for multiple items-that never crossed my mind.

I take the point that some sellers may see it that way, just saying that isn't the impression I would guess that most buyers have.

But what it comes down to is a seller can knock back any offer anyway.

Have to say though that in the rare cases where I have made offers, they have all been in single items and all have been accepted, but I usually offer close to the asking price, with perhaps a 5% discount.