Good post , however I didn't get the point of purchasing power of seller, can you elaborate more what do you mean by that , and how it can be a benefit for me ?
When you are starting you cannot compete with big sellers. They will most likely just undercut you if you try to sell what they are selling, therefore you must make sure you are only competing with smaller sellers with less buying power. Less buying power means less likely to be able to undercut you. Its the same for any business whether small time ebay side hustle or massive corporation. You can get a better idea of who you are competing with by looking at their address and their feedback.
I agree Ebay and online selling in general is becoming more competitive and more serious , every one try to shortcut the supply chain, and your only bet is becoming the good customer service (which amazon is focusing on ) . if you have such good rating 50K , I would not retire that account , at worst I would sell it to others , as getting all that positive feedback takes time and efforts.
Good customer service is great if you are selling consumables where you expect repeat business. However online market places like Amazon and ebay offer very little customer loyalty because the nature of the market place is to always be offering the best deal. So be careful not to assume just because your customer service is good you will get repeat business. It's primarily the lowest price, then coupled with free delivery (even though its not really free) and a no quibble refund policy that will get you customers. But like i say, lowest price is number 1 selling factor.
Regarding selling the ebay accounts its only possible if you are selling the account as part of a limited company.