There used to be an add-on for eBay.co.uk that really helped out buyers and sellers. I have recently come back to firefox and could not find the add-on anywhere. No GM script was the right one. Then I found one but it was for the German or US eBay site only.

This is an adaptation of the GM script that was an adaptation of The SellerBrowser.
We needed a working version of this for UK users!
The original script see here: http://userscripts.org/scripts/show/8506
- only worked for Germany and USA eBay sites.
Well now we have one for eBay.co.uk after much deleting and pasting!
I will post details for use soon but you can get the idea from the link for the version I modded:
http://userscripts.org/scripts/show/8506

Basically, once installed, when you are on an ebay page, highlight any ebay username on the page. Then go to the greasemonkey icon in status bar of browser.
you'll see a menu there for 'User Script commands...' Then 3 different options to launch the Seller Browsing.
Try them out.
I recommend just using the menu option there that is titled 'SB Browse Member' as the others are not that good but I just haven't worked out how to remove them yet!
When you launch this, Tabs will open for each part of the investigation of the username you highlighted. (By the way, if you do not highlight any eBay username and then press the menu option as outlined above, you will get a popup menu asking you to select a username that is found on the page).

The Investigation of the user gives you the following details about that user:
'Feedback',
'Items they sold that have ended in the last 31 days',
'Current items for sale',
'The Users Postings discussion boards',
'Postings discussion boards containing the Users name',
'toolhaus.org - Negative Feedback enhanced tool',
'Negative feedback for the last 12 months',
'Neutral feedback for the last 12 months',
'Recent auctions User has bid on',
'A general Google UK search on their name to see if any info comes up',
'The Users Completed Items',
'goofbay Seller History',
'goofbay Bid History',
'Member search',
'Member User ID History',
'My World page'