That might be an indication that your browser is not handling cookies properly. If there is nothing odd about your network and you are not using any privacy software such as Ghostery or Disconnect, then most likely either you are deleting eBay cookies, or you have a corrupt or blocked eBay cookie.
Try manually deleting your eBay cookies once to see if that makes a difference.
Or try another browser.
I just tried going to my purchase history on ebay.ca. Although I had to sign in again, there was no Captcha and I was able to view seller profiles from my order details.