NYC Transit Station Signs
Take a piece of Times Square home with you, or West 4th Street, Neck Road, or any other number of subway stations which have signs for sale. Some signs are less than $100, but Times Square will run you $2,500.
Subway Tokens
Before there was the ubiquitous MetroCard, straphangers paid their fares with tokens. While they may be absent from the system now, you can find plenty for sale from the MTA, at antique stores, and some are even made into cufflinks. (If you look closely, some turnstiles still have slots where tokens were once accepted.)
Subway Seat
If you can't manage to get a seat on the train each morning, take a piece of the train home with you and always be guaranteed an open spot. See the actual seat for sale here.
Refuse Canister
Otherwise known as a trash can. Hopefully they are cleaned before being put up for sale.
Grabholds and Stanchion Poles
While seats and decor clearly have a use outside of a subway train, these might take some more creativity to put to use around your house.
Subway Strip Map
The 7th Avenue line got new strip maps recently, and the old ones are now for sale. You'll never forget the 2 train's route with this on your wall.