I guess the term "lowballer" refers to people that have no idea or have not done any due diligence on the market asking price and just tries his luck and shoot a low (and I mean really low) unrealistic offer.
Sometimes, a seller might priced his for sale items at above market value... but in the end, it's a buyers market.
When an item is left unsold at the sellers high listing price, then sooner or later he will understand that the asking price is just not what people are prepared to pay or even realistic in the market asking price.
When a thread says "offers welcomed", then be prepared to listen to all offers no matter how low.
The worse that can happen is to reply "no thanks" to the offer.
"Time wasters" refers to a person that have verbally agreed to a deal, price have been agreed on, and while awaiting payment, suddenly pulls out of the deal with some last minute concocted story? I dunno.
This is just my take on things.
I have sold and bought many watches and also other stuff online and I have came in contact with many many colourful characters...