Hey there, thanks for the review! ;)

Normally, after you use the search bar on a website, you can look at the URL and see where the site placed your query. For example, if you search for "Firefox WebExtensions" on Google, Google adds "q=Firefox+WebExtensions" somewhere in the URL (among probably a lot of other things). "q" is the parameter Google uses for user queries.

SSS only needs a URL that says "{searchTerms}" in the part where your query usually goes. In the example above, that would mean the entire URL but with "q={searchTerms}" instead of "q=Firefox+WebExtensions". Your URL does not have a {searchTerms} part, so SSS doesn't know where to add the text you selected. :)

You will notice that many websites use the ? symbol and then add a lot of parameters separated with the & symbol. You'll usually be looking for the parameter that represents your query and replacing that with {searchTerms}. Most of the time, the URL also contains a lot of unnecessary parameters, so, to give an example, this simple version is what I use for Google:
https://www.google.com/search?q={searchTerms}

eBay is actually weird! It has several different search URLs, some of which very strange, but from searching around a bit it seems this one is simple and works:
http://www.ebay.com/sch/{searchTerms}

This is a bit technical but I hope I have helped. :) If the explanation wasn't clear enough please tell me. Maybe I should improve the options page instructions with a similar explanation.

Cheers!
Daniel