Yes seat belt extenders are legal to buy and legal to wear.
When you're larger and cannot fit into the regular seat belt then a seat belt extender is needed so that you can wear your seat belt which is the law in all states.
If you do not wear your seat belt and get pulled over you can get ticketed even when the seat belt won't fit you.
However if you cannot fit in the seat belt then you need to have a seat belt extender and as long as they are properly installed the seat belt extender is legal and safe to wear.
If you do need a seat belt extender do get one and install it so that you can safely wear the seat belt as they do save lives and can keep you from being thrown from your vehicle in the event of a car crash.