Many of the earliest US issues were cancelled with pens. From 1847 to about 1865, not all post offices had postmarks. Larger post offices that cancelled lots of mail had postmarks. But smaller offices with much less mail usually didnít have a postmark. Postmasters used a pen to cancel the stamp. The effect was the same; the stamp was cancelled to prevent reuse.
The Scott catalog prices pen cancellations on issues prior to 1865. While pen cancellations continued into the 1870s, their value is much less. From 1847 to 1865, pen cancels are worth about 50% of what a stamp with a normal cancel is worth. There are some variations from this rule. For example, Scott #39 catalogs at $10,000 used, but only $2900 with a genuine pen cancel. Fifty percent is a rule of thumb though.
After 1865, the Scott catalog doesnít price pen cancels. On these issues, pen cancels can bring 20% or less of catalog value. For example, Scott #65 is very common. Many copies have normal cancels. Pen cancels on #65 are worth very little under normal circumstances.
Itís all about demand. Collectors prefer normal cancellations over pen cancels. Pen cancels arenít as attractive, may be heavier in application and ďa pen cancel isnít a real cancelĒ in the eyes of some collectors. Thatís why they are worth less.
Pen cancelled copies are an affordable way to pick up some of the more expensive issues because they can be obtained for much less. After 1865 though, I would avoid them. Collectors seem to be more forgiving on the very early issues. They are more receptive to the idea of a pen cancel on those issues. After 1865 though, collectors tend to shun pen cancels on all issues.
As always, collect what what you want. Financially speaking, you should be paying less for pen cancelled copies of stamps. When selling, pen cancelled stamps will not be worth as much due to lower demand.