When people are selling their products privately on some online websites, I often observe, they write VB with the price. Can somebody tell me, what does VB stand for and what does it mean?
In that context, "VB" means "Verhandlungsbasis". The listed price isn't fixed, but the basis for negotiation. The number is meant to give you an idea, a ball park number, but the details and the final price is expected to be negotiated between you and them. Sometimes, there isn't any number given, just "Verhandlungsbasis". That's roughly the equivalent of "Contact our sales team!" in English language offers.
As Michael A. Schaffrath mentioned correctly in his comment, you see "VHB" as an abbreviation for "Verhandlungsbasis", as well.
Verhandlungsbasis, it means that the given price is negotiable.