In the case of ebay at least, the normal ublock origin seems to prevent this (maybe just incidental that it blocked the loading of the port scanning script?)

Open “web developer tools”, “network” tab and browse to ebay - if uBlock Origin is turned off, after a few seconds you start to see lots of websocket connections as is shown in the article here. With uBlock Origin enabled, I’m not seeing those.

EDIT: Raymond confirms this ^{[reddit link]} and asks for some ideas on how to specifically block malicious connections to localhost