Hello Janice & WELCOME to these Forums....
Since you are trying to identify your particular tractor this forum should work well, there are quite a few folks that repair various models follow here...
If you don't have a picture you could post, go to Ebay and do a search for Gravely it should turn up a lot of models, try to find one that looks similar to yours... it will also give you a general idea what they are selling for, $100 is way too little for any operational gravely, the tires & wheels alone should be worth that....
Is yours a 2 wheel or 4 wheel rider? It could also be a zero turn, Gravely has made various models...
Those older Gravely's are better built than newer models so it's age is not necessarily going to reduce the value.
If you can find a serial # this attached pdf may help in finding the year:... :smile:KennyV