ipad

You are using the id that you were using in Settings > iCloud on the iPad, the iPad shows in the Find iPhone section of that account on http://www.icloud.com ? If not have you tried : If you forgot your Apple ID - Apple Support ?

Or, and if you were the original purchaser of the iPad (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it

If you can't find the id that you were using in Settings > iCloud on it try contacting the store that you bough it from and see if they can issue a duplicate

Does the store not have an email address or contact details ? Without proof of purchase (i.e. receipt with serial number on it) Apple won't be able to potentially remove activation lock