Basically exactly like the example "$100 for this listing? Can I offer $20 cash?", you put a reasonable price and the low baller makes an "offer" of something extremely low.
Happens all the time with Gumtree, no matter what you list for sale and how reasonable your price is you will nearly always get some idiot that thinks $5 is a good offer on your $100 item.
Think of it like someone selling a brand new MISB TR Trypticon for $100 and then getting constant "offers" of $5 and you'll see why lowballers get to be annoying to genuine sellers.The arguments I've heard scalpers make to try and justify their prices can be quite laughable.
"Oh this is no longer in stores so that automatically doubles the price, it doesnt matter that it didnt sell back then and I got it for 5 cents, the fact that it's no longer sold now and that you want it means it's rare and rare = valuable"
Don't even get me started on the ones that buy every copy of an item that they can to artificially make it "rare" and "hard to find" just so they can inflate the prices, absolute scum of the earth and one of the reasons I never got a nes mini and why my mate missed out on ordering a snes mini, because neither of us will pay some jerk $500 for an item that's barely worth the $100 RRP.