Wayne Gehret in Ephrata, PA may be the best option for low denomination postage:
https://www.usmintsheets.com/I buy postage for mailing, especially small format stamps in booklets, or resale on
eBay at competitive rates, but the latest spread for USPS First Class packages is $3.50 retail (stamps) or $2.68 printed postage may force use of labels. I expect that when
eBay takes control of payments in 2021, they may try to force sellers to use
eBay labels instead of stamps or third party labels for mailing
eBay lots.
When I have a surplus, I offer bulk postage on
eBay in lots of about $500 face at auction starting at $9.99. The stamps are clean, never hinged and carefully counted. Scrap is sorted by denomination and sheets are inventoried. Realizations had been in the 55% to 75% range, but may continue to drop.
Bel-Kirk Stamps in Kirkland, Washington sells postage on
eBay in $30 dollar lots. Check his auction realization on
eBay sold items view. The user ID is bel-kirkstamps. They will pay a fair market price for postage if you are within driving distance of Kirkland, Washington.
http://stores.ebay.com/belkirkstampcoinandcomics