Yes, you need a permit for your yard sale. 

Before you have a yard sale at your home, you need to get a permit from the York Police Department.  In fact, anyone who has a yard sale, porch sale, barn sale, garage sale, auction or flea market at their home is required to get a permit.

Sources:
Ordinance for Yard & Barn Sales Zoning Ordinance-Article 16 (Signs)

Frequency and duration for sales:
Each residence is permitted to have up to two (2) yard sales each calendar year.  Each yard sale can last up to 3 consecutive days.

Register in person at the Police Station.
Before you have a yard sale, you need to register for your permit. 
You may do this online OR in person at the dispatch window inside the front door of the Police Station (36 Main Street, York Beach).  They have a form for you to fill out, and they'll check to be sure you don't have too many yard sales per year.  They will also give you the permit, which you should have with you on the day of the sale.

You can register at anytime of day, any day of the year.  Best of all, there's no fee.

Safety:
The Police Department has the authority to shut down a yard sale if it is causing vehicle or pedestrian safety problems.  This isn't common, but it has happened.  If you live on a busy street or have any concerns at all, as the Police Department for guidance and assistance ahead of time.

Signs:

In addition to placing an ad in the newspaper, people attract customers with signs.  In York, all signs are regulated by the Zoning Ordinance.  The Town treats yard sale signs as temporary signs which do not require a Sign Permit.  Still there are some basic rules which must be followed:

Not more than 4 signs shall be located off-premises;

Off-premise signs shall not be larger than 5 square feet;

All signs shall be posted not more than 24 hours before the start of the yard sale, and shall be removed within 1 hours of the end of the yard sale; and

No sign shall be placed such that, in the opinion of the York Police Department, it constitutes a traffic or other hazard.

Signs which violate these standards may be removed by the Town.