Well both of those questions swing both ways. Legally you are suppose to pay tax on any income, however there are thousands of yards sells everyday and most all do not pay any taxes, just depends on the person..
Next, as far as advertising where the items came from, it really depends on alot of things. Most major dept stores require their labels be removed before selling their liquidated goods. Some branded goods require their label to be marked through to prevent returns and refunds.
You have to be careful about how you advertise so as not to violate anyones trademark, copyrights and such...