Well for one, $500 had better be getting you a very lightly used Black Widow in very nice condition! American Sportworks, the brand that manufactures them, does advertise them as "Made in the USA", but remember that they note quietly "with imported components". Basically they take Chinese made components and assemble them in America.
As far as I know there's nothing inherently wrong with them, but I would recommend tracking down a true vintage yard kart. They're generally very solidly built, extremely mechanically simple, and have a lot of universal parts. Something like those Black Widows will have more proprietary parts.
Are you going to buy both, or just one?