do not offer a lower price than what is listed/asked-for until you have actually gone to see the item.
there are exceptions* to this rule so err on the side of not being annoying and just go see the thing and negotiate with the seller then. even if a seller lists it as OBO, your chances of getting what you want are better if you show up with moola in hand and then offer less. it’s hard to say no to that.
*i totally broke this rule once for some diapers and was so scared and apologetic to do so since i HATE when people do it to me. thanks for being understanding, robin!