USPS will accept any US stamp in exchange for another stamp no matter how badly it has been defaced (even if they got wet; faded, smudged, torn apart or stuck together) providing they've not been bumped and you follow their instructions to glue them on a sheet of paper to inventory.... The will not refund cash; only exchange.

I've done this several times with those old sheets found in boxes of paper at flea markets and whatnot... Also, if I remember correctly; they'll bump everything on the sheet before they make the exchange...

JB