My watchmaker called to tell me that one of his clients (a high end family run jewler in town) had gotten a ladies 60's Seawolf in and the customer wanted to sell it. He knows I have a thing for the Seawolf and would love to get one for my lady a his/hers set (we always joke about it).
The jeweler reached out to me and told me that the customer wanted $1.3k for it, I pleasantly declined and said that's ok, thanks for your time. He responded saying that he could pitch her an offer, and asked what it is worth (these aren't in their wheelhouse). I told him I didn't want to insult, but then laid out 2 paragraphs of comps and research so he understood I wasn't trying to insult him (as he is friendly with my watchmaker- I care about that relationship), but was giving him real market value so he understood the customer was not informed of the actual value- I offered $250 (which is top of the market for that kind of ladies watch- but hey, the lady wants it) and would give him a 20% finder's fee for his time.
He thanked me and said he would relay that to the customer. I assumed it was done, but yesterday (2 months after this interaction) he emailed and said she took the $250. I pick the watch up today.
If you just throw a price out to an uneducated seller, it can look like you are just being insulting. If you share your rationale, then they understand you are ernest and can meet you on a more honest playing field.
As Fred said, in person you can read the body language and get a feel for how the seller wants to do business- just the facts and give me a price, or so they want to kibbitz and feel you out before they get to the money part (all part of the foreplay sometimes). It's much tougher online and all depends on what kind of relationship you want.
With eBay and a one off sale with a "make offer button" I will just throw it a low ball sometimes and see if they bite. Second offer will be the real one.
If I am asking for them to take an auction from open to a BiN for me, I will send a 1/2 sentence DM and ask politely with a real value- many times they accept.