Go thru the RESOLVED Problems with 2.6.4 thread.
I had a personl paypal acct. i was told to remove all ipn info from paypal and i did. But i suspect that created problems with paypal. I opened a brand new paypal business account (its free to open, but i think they will take a small percentage of your sales). I DID NOT touch the ipn area or change anything else on paypal. I only filled in the info necessary to open the account. Once i had the account open, i made a test purchase and it worked.
You are allowed to have one personal and one business account with paypal. Maybe you are having the same issue?
If you open a new account, be certain to change your email address in the symbiostock settings area.