There is more demand for U.S. back-of-the-book stamps than regular postage stamps as many collectors build a collection of regular postage sections and then go on to the back of the book.
Stamps produced for the U.S. Postal Agency in China are popular because of their low issue numbers, limited period of use, and demand among both Chinese and US collectors. The 24¢-on-12¢ claret-brown Benjamin Franklin stamp is a good one to look out for. Only 8,000 such stamps were issued and there is a growing demand for them among collectors.
The stamp is valued at $100 in unused hinged condition, at $260 in mint never-hinged condition, and at $225 in used condition by 2018 Scott Specialized Catalogue of United States Stamps and Covers. Considering the rarity and demand for these stamps, they seem to be undervalued, making them a good buy!