how do you find a ebay id

[eburtonlab]

If you know a seller's username, you can search for that seller's items for sale using a desktop browser to go to the Advanced Search page and performing a search by seller:

[woodland_gnome]

What information about the user do you already have?   

If you have his/her contact information (either directly or searching the web or Yellow Pages or the like) you can try contacting  him/her and asking.