Actually your question should have been that while journalising 'should I credit Purchase A/c or Purchase Return A/c on returning back of goods purchased to the seller'. I am saying so because while doing posting you debit or credit only those accounts which have been mentioned in the journal entry. Let us now understand it:
Whenever goods are bought, they are debited to Purchase A/c and whenever any goods are returned to the selled 'Purchase Return A/c is credited and not Purchase A/c'. The reason for this is that for management purposes and for easy settlement of accounts of creditors, it is better to keep two separate accounts for Purchase of goods and Return of goods. So if you while journalising maintain separate accounts for purchases and purchase returns, obviously while posting you will post it two different accounts.
Same applies to Sales and Sales Return A/c.