Especially as the amount they offer is never anywhere near what the thing fetches in the end.
That's waht I was going to say, and that's the OPs answer. Your offer is waaaaaay too low, basically.
That said I pulled a blinder on a bike I bought a few months ago:
It was advertised as "Readvertised due to time waster" and it was collection only, about 3 miles from my house. Seller obviously wants shot of it.
So I message him about doing a deal off eBay, and we arrange to meet. I have a good look at it, seller is friendly and genuine, as am I. I make an offer which he accepts. Everyone is happy.
BUT he didn't know that I know EXACTLY what it 'sold' for first time and I just offered him a bit below that figure, pretty much knowing he just wanted rid of it.
I took the bike home, gave it a FULL strip, clean and service with all new cables and more then doubled my money on it. It was a 1994 Kona Hahanna