There is no magic formula unfortunately. It all depends on two main factors - how “fairly” do you think it is priced and how badly do you want it?

OBO simply means that the seller accepts offers and is willing to negotiate. I does not mean that the seller has to accept anything. Its not like there is a timeframe where any specific offer must be deemed “best”.

I once listed games as “firm”. Thought it was making it easy as i was telling buyers what i was willing to accept and all they had to do was decide if the condition of the game was worth that price. A lot of people however feel the need to negotiate - regardless of the price - as they feel the sellers have left room intentionally for just this occasion. Bottom line: a good deal is only a good deal when both parties walk away satisfied.