Tinkering with technology is my hobby, so I spend a lot of time on secondary markets (like Facebook Marketplace, OfferUp, Craigslist, eBay, etc.) I always research the original price of any item I’m interested in, any current discounts being offered by retailers (like Amazon, etc.), and what the average pre-owned price appears to be across platforms. Then I offer some amount less, and expect a counter. I equate negotiating with fishing. I know that, with patience, I will eventually find a motivated seller. So I try to never enter any specific negotiation needing to make a deal. Obviously the rarity of an item and your desire to obtain it are big factors that will influence the price which you are willing to pay, but knowing the item’s value and being unafraid to make a reasonable-but-low first offer has proven successful for me. Also be courteous. I have had better success providing context and asking questions about the item then merely opening with a low effort bid, and have even had several sellers return to the negotiation hours, days, or weeks later because I invite them to if we are unable to reach an initial agreement.
It would be much simpler to use an external battery pack, like this.