What is "low ball"

I would say it depends on what the buyers asking price is. If the asking price is close to what other similar items have sold for recently, then approx. 20% less than that amount or more might be considered "low". If the buyer is asking say, $1000 for an item that usually sells for $800, then an offer of $750 would not be considered a "lowball" offer, because his price is too high to start with.

lowballers = bottom feeders

Lowball is in the mind of the seller.
If the seller thinks the offer which is below his or her asking price is a lowball offer, than it is such.
I would say any offer below the asking price in an ad mentioning no lowball is going to be considered a lowball offer.
The closest to making a less money deal with such an ad would be to ask for free shipping. Even that would be an insult to many with such a blurb in the ad.
So Lowball is anything less than the asking price to the seller mentioning the phrase.

As for the potential buyer.. Why bother even making any sort of offer under asking price, to a seller with such an ad?
It is clearly a waste of time.
So in that sense, anyone who WOULD make an offer below asking price to a seller stating no lowball offers, is just being annoying, and thus deserves the title of 'bottom feeder'.

Lowballers=bottom feeders? I don't think so. It's the buyers who set the sales price not the sellers. I have been tracking sales prices on certain items and it is the sellers who are continuously trying to jack the prices up. So, if your price is too high and a buyer offers the going price, is the buyer a lowballer? I don't think so.