Taxes: Some, all or none at all

Hopefully the various tax departments never see this but... we're somewhat surprised by the differing sales tax policy of thrift stores.  Some stores charge only PST while others charge GST, some charge both and others none at all.  Some take cash only and issue no receipts.  I'm not sure how the accounting is done but I am happy not to pay any taxes.  Perhaps it is their status, or they don't sell enough?