This is a constantly changing list of the most valuable Poland stamps and overprints with proofs of sale.
$63,250.00. October 2009, Cherrystone auction. Poland 1919 10 kr blue, overprinted in black on Austrian Empire stamp, position 17, both “P” letters in “Poczta” and “Polska” damaged.
$57,500.00. October 2009, Cherrystone auction. Poland 1919 6h orange, Form I, position 14, thin “Z” variety, overprint inverted, never hinged.
Where to buy Polish stamps?
https://thephilately.com/listing/poland – Polish stamps for sale by ThePhilately.com