The USPS does not buy them back. Most stamps sell for below face value on eBay. If you mail packages/letters a lot, just use them for postage for the maximum return. I tend to plaster them all over USPS fixed price boxes using stick glue.
Full sheets of stamps can sell above face value on eBay, but the buyer will expect no blemishes, creases, or wrinkles on either side of the sheet. The amount you get over face for the issues pictured will not be all that much and eBay fees will probably cancel out any gains.
I used to buy modern/old mixed collections on eBay, break out anything of value above face, then use the rest for postage on eBay items that I sell. Not hugely profitable, but a fun way to acquire the occasional valuable older stamp (pre-1920) for very little money.