The most important item ever printed in Lancaster County is the Articles of Confederation, printed in 1777 by Francis Bailey. (Although, for me and for many bibliophiles, the Ephrata Martrys Mirror and the 1819 Baer Bible are personally more interesting.) Bailey's Articles of Confederation would certainly be the most expensive item ever printed in Lancaster County, if Williams College in Massachusetts ever sells it.
Francis Bailey printed 300 copies of a 26-page pamphlet that would become the greatest legacy of his printing career.
This document was the first constitution of the United States. Bailey printed this work for members of the U.S. Congress, who were meeting nearby in York, where Congress had relocated from Philadelphia, because of the British occupation of Philadelphia.
On November 15, 1777 the Continental Congress has approved these Article, at York. Congress instructed Bailey to print 300 copies. So he did. ....He printed a beautifully-ornate document that is now one of the most important founding documents of the United States.
Other printer had printed early, preliminary versions of these Articles in 1776, but Bailey's printing was the first printing of the completed Articles. Later that same year, other printers throughout the states printed their own editions.
Today there are only a few surviving copies of these Articles, including preliminary printings from 1776, and completed ones from 1777.
Williams College has one of the Francis Bailey printings. The college library display's Bailey's document in an exhibition hall, in a display they call Founding Documents of the United States.
The college web site says "The Articles of Confederation are the most sumptuously printed major American document of the 18th century, and of the nine extant copies none is more perfectly preserved than that in the Williams College shrine of the Founding Documents."
The Williams College exhibit, with Bailey's Articles of Confederation, is here.
An image, with history, of Bailey's Articles is Here, at Philly's Pennsylvania Historical Society.
A good history of the printing of these Articles of Confederation is here.
Later that same year, in 1777, William Purdie printed these Articles of Confederation in Williamsburg, Virginia, as shown Here. This is one of the few surviving copies of the 1776-77 printed Articles, with Bailey's being the first official / complete one. (And Sorry, Mr. Purdie, but your edition is rather boring, compared to Bailey's.)
The National Archives' information about the Articles of Confederation is Here, with a link to the manuscript edition that is signed by all the state representatives in 1781, when the Articles were finally ratified, to be the first constitution of the United States.
In 1787 the U. S. Constitution replaced the Articles of Confederation as the official frame of our government. That Constitution is Here.