Definitely a 79 if it is Spark Yellow. YU paint code only available in 79 model year unless someone painted it(doubtful). My guess is it that it is an early production car,probably built in 78 like mine. I say offer as high as $500 if the body is clean but a non-runner. The pic makes it look pretty rust free,but it can,and will hide on any RX-7 of that vintage. Look around the idler arm on the frame rail they tend to rot there as well.
Look for a 4 or 5-speed to tell S from GS as well as a two or four-spoke steering wheel.Although that is easier to move from a GS to an S so not always a giveaway for the lower model. The 4-speed two spoke steering wheel cars are the only real way to tell an S from a GS on the early cars as the AC,and sunroof wasn't always put on the early cars as Mazda was pumping out as many cars as they could early,and the lower optioned cars were easier,and quicker to make.
I love my Spark Yellow early production SA,and wouldn't trade it for the world. You can't go wrong with these cars if they are fairly rust free. Stupid simple,and easy to work on. Just don't rice it out please these cars deserve a better fate than that.