Hi, my experience is with HPs, but I was definitely able to run them in my storage unit (as well as
in the back of my truck) by using a portable power pack. The one I used was an automotive "power box" (sold at Canadian Tire up this way) with a rechargeable 12V battery. I had a new lithium battery in it that replaced the original lead acid one, and it gave me at least a couple hours to run the printer, doing test plots, head cleans, etc. Pic in storage attached.
And even then, there was still an electrical outlet not far down the aisle (I used it to charge lamp battery while there), so in a pinch an extension cord might work if you hunt around for an outlet.
I can't help much with the printer itself. For my experience with my HPs being in storage "more than once", it wasn't a huge deal despite my worries -- they always came back to good working shape eventually, and that was 1-2 yrs storage at a time. Being unpowered for an extended period is a concern, and certainly a good negotiating point, but it doesn't necessarily kill them. It does take its toll on HP printheads tho, so it definitely carries a cost.
My view is if I can't run/test it to at least see the onboard diagnostics/service info, then the most I'd pay is like salvage/disposal parts price, very little. I mean I'm shopping for a printer not parts. If the seller wants more, they're going to have to find a way to let me see the operating condition it's in. It doesn't fly if they're asking the same price as somebody else who can demonstrate theirs running.
And the amount of good supplies included factors into my risk willingness too. If Epson inks etc are as expensive as the heroine-laced stuff HP sells, then I definitely want to know how much I'm getting. And if I have no other printer that can use those same supplies in case the printer is a dud, then they may be worth very little to me.
I'm not suggesting this seller is trying to scam, I mean there's a lot of Life(tm) happening to everyone right now... (my own were stuck in unplanned storage for 18 months!). But we want to be sure we're getting something that will work for us, within whatever amount of money and effort it's worth to us.
Hope it works out for you either way!