I'm not clear what you mean by contacting a buyer "on another account"?
If you have been in a recent transaction with a buyer you can request their contact details here: http://www.ebay.co.uk/sch/ebayadvsearch?_sofindtype=9 It has to be the account on which the transaction took place, not on another one.
Otherwise you can only use eBay's messaging system, which they can access through their account on any connected device.
