Signing the label is destroying it just as much as writing "little Timmy" on it. If it's a copy of Adventure you have signed, it may increase the value cause hey, it's Warren Robinett, and Adventure is cool. Same for having HSW sign E.T. You're using a cart vs some other material to write on, so the value goes from maybe a few bucks for a crappy cart to whatever an HSW autograph costs + crappy cart.
On an actual collectible cart with some kind of real rarity and money value you're screwing yourself. Fake autographs are a real problem in the memorabilia business, and frankly, I wouldn't pay any real money for one that isn't authenticated. Many, many people still won't care who signed it. You screwed up the cart.