I just bought a used 99 altima and when I decelerate from any speed above 65, the car shakes alot. does anyone know why that happens?

 

Warped rotors. Have a good brake place check the discs (they may need to be replaced).

 

brake rotors on ebay are about 80 dollars for two front ones, and pads are super cheap, normally under 60 dollars, and any dope good with cars can change them in about 30 min to an hour, but yea, take it to a brake place n see what they want to charge you, its super easy to change brakes, ive done it like 4 times already on all of my friends cars, very simple stuff, hope it works out for you