So here are the only game pieces that are actually very valuable. Don't trade these away:

BROWN: Mediterranean Ave, odds 1 in 30 million wins $1,000.
LIGHT BLUE: Vermont Ave, odds 1 in 40 million, wins $5,000.
PINK: Virginia Ave, odds 1 in 200 million, wins $10,000
ORANGE: Tennessee Ave, odds 1 in 602 million, wins Super Bowl tickets.
RED: Kentucky Ave, odds 1 in 15 million, wins one of 40 plane tickets for two.
YELLOW: Ventnor Ave, odds 1 in 300 million, wins $20,000
GREEN: Pennsylvania Ave, odds 1 in 40 million, wins one of 15 Fiat cars.
BLUE: Boardwalk, 1 in 602 million, wins $1,000,000 in $50,000 annual payments.
RAILROADS: Short Line railroad, 1 in 150 million, wins a year's supply of gas.

So, if you find one of those pieces, you're actually special. If you find any other piece, it's basically worthless, unless you can find a guy with an actually valuable piece to sell it to