First, go to Bookmarks, then Quick Searches, then right-click any bookmark (you have to because there isn't any empty area otherwise, also I recommend you right-click the topmost bookmark to find later for maintenance). For this example I will use toolhaus.org's ebay negs tool, but more are posted below. I chose the bookmarks name as "toolhaus.org ebay negs", the Location is
and the Keyword is "th", as seen in the image.
Then, to access a person's negative eBay reviews and feedback you simply type 'th [username]', where [username] is the persons eBay username. Also, you may want to add a description in case later you might forget why you created that particular Keyword Search.
You can create your own Keyword Searches for any website that uses parameters in the URL, and replace the search term with %s. There's more Keyword Searches at the MozillaZine Wiki.
For searches which require forms the NeedleSearch toolbar can help in this regard. If you prefer to use browser based bookmarklets, there's two at smallbusinesssem and many, many more on JesseW's Wikipedia user page.
Here are some more searches which I've added:
|Google Groups|| ||gg|
|Google's 1st 100 results|| ||g|
|Sourceforge Project page|| ||sfp|
|Wiktionary Lookup|| ||wk|
|Google Books|| ||b|
|Discogs Artist|| ||da|
|Alexa Rankings|| ||alexa|
|Web.Archive.Org Wayback Machine (this one doesn't work)|| ||wa|
|Google Addurl Quick (lol, this doesn't work)|| ||add|
|eBay Item Number|| ||ebi|
|Amazon.com New&Used ASIN (asinf [asinf])|| ||asinf|
|Amazon ASIN Featured Seller (KW: asin)|| ||asin|
|Google Images|| ||gi|