When does seller's responsibility end?

I would say the seller's responsibility ends when after plugging it in/setting it up it works as described.  To be generous I would give the buyer 1 day assurance that all is well.  It is the buyer's responsibility to immediately check it out upon arrival.

"....I would say the seller’s responsibility ends when after plugging it in/setting it up it works as described. To be generous I would give the buyer 1 day assurance that all is well. It is the buyer’s responsibility to immediately check it out upon arrival..."

+1 ....full stop.

OP fact circumstance as described with an intervening month until Buyer’s sudden wailing started sounds more IMO as either
(a) buyer’s regret,  or
(b) buyer f****ed it up after he played with a month.

so ..... sayonara and tuff noogies to Buyer and no further seller responsibility(ies) ....full stop.

After 4 week´s correspondence with eBay and after 4 reports on buyer´s inappropriate behauviour eBay finally ended the case for my favor. Every time I replied them I referred to my ad and the photos in it. They had nothing to hold on. One can´t con an honest man.
Yes, eBay really LOOKED at my ad again and said it wasn´t my fault and my money was released. Buyer was accusing that I hide marks/imperfections on the item. Even most of the biggest marks/scratches are actually visible in my ad. Of course it wasn´t my fault, I knew exactly what I had been doing all the time, buyer just tried to cheat and crab money from me but failed. In fact, he had earlier before sale´s end suggested that I could pay half of shipping that would be very expensive for a 45 kilos parcel and the actual cost was still unknown. At this point I went suspicious but agreed to pay half, as a gently gesture. Funny, he never thanked me about this...

After sale buyer made several basic mistakes, not accepting return, demanding "extra services after sale" and "Misused the programme to claim an item was not as described". In frustration when he obviously realized that eBay won´t play the game he had planned he had left a negative feedback. His biggest mistake was "leaving an inappropriate feedback" when case was still open/unsolved. Furthermore, eBay removed that negative feedback right after.

Great, there´s still common sense left but they held my funds for four weeks. That was really bad because I really have no other funds at the moment. I told them that my economy is seriously harmed.

I learned my lesson. I immediately withdrew my money to my bank account and never will sell anything expensive there. And won´t be PainPal´s fool anymore. What a bloody waste of time.