I need to add a bit more.
If you followed my instruction to type 'eBay' into the search box you will have gathered all the emails to and from eBay.
But. You'll also have any emails in which you mentioned eBay. Not so clever. So search instead for ebay.co.uk and narrow the field.
And to further add - unless you're hopelessly wedded to folders you can now see that it's so easy to search and group emails there's really no need to 'folderise' them. A good reason not to might be that by running the earlier and broader 'eBay' search and seeing them all in chronological order you can then see when you bought that thing and just above it will be the note you wrote to your friend telling him that you'd bought one, his reply etc.. which might be handy.