I'm thinking of buying black carpet for my 71 rs an a guy I know had a new one for half the price but it was for an 80 will it fit my 71 camaro? Thank you

 

The '80 had a catalytic converter, so there will be humps formed into the factory carpet that your '71 floor wont have. It will fit, but might take some work with a carpet steamer to get it to lay down.
Carpet sets usually go under $135-$145 for basic original formed carpet. Not sure it's worth the savings to fight getting it to look correct?
70 74 for GM F Body Carpet Set 1 Piece or 2 Piece Style 80 20 Loop Color Choice | eBay