You can set your feedback to PRIVATE but you won't be allowed to sell with private feedback only buy.
The best solution is to have two seperate accounts.
One for buying ( with private feedback )
One for selling ( you can make the items private when you list )
There is a link on this page to set your feedback to private.
http://pages.ebay.ca/help/feedback/allaboutfeedback.html
There is a link on this page that explains how to list items in the private format.
http://pages.ebay.ca/help/sell/private.html