I make custom vinyl decals. is it illegal to make a sports team logo and sell it to someone to put on there car.

Generally speaking, yes. The unauthorized and/or unlicensed use of trademarks is likely an infringement upon the rights of owners of the intellectual property (typically either the teams, the league, or the school). There is recourse available for the owners of such intellectual property against infringement, and it is not uncommon for rights holders, particularly in the realm of professional sports team logos and the like, to proactively protect their marks and enforce their rights.

Almost always forbidden. Always for professional sports teams.

It's illegal to make and sell an unauthorized decal whether or not to put on a car. If the sports team or the league trademark people find out, you can expect to get stopped and/or sued, and you'll lose.