I was appalled to see among "My Summary" items id'd as sponsored items due to your recent viewing [...] that turned out to be listed by the same seller that listed the historic posts that I had viewed earlier
eBay is likely suggesting other items from the same seller that you had viewed items from previously in this case. eBay will also suggest other items from saved sellers or items that are similar in some respects to other items you have searched for in various ways.
Some of the items suggested on the main ebay.com page can be erased by going to your Recently Viewed page and using the "Clear all" link there; that may not clear all the suggestions, though.
Other ways to avoid such suggestions: remove the saved seller from your Saved Seller list -- if those suggestions were from a particular seller on the list. Perform new searches repeatedly to displace the old ones that the suggestions are drawn from. Or use a different page as your landing page to avoid the suggestions. Or use an ad blocker with an element hiding tool to block those suggestion rows from appearing at all in your browser.
Also possible options to avoid this in the future: searching using a different eBay account or while not signed in to your account, though this would make it much harder to save items to a Watch List or place an order.
