what version is it. Model 21 came in A, C, E and maybe more. I have an A and a C. If yours is very early, it's probably an A.
If you have all the accessories, make sure you don't loose them. These came with a set of cams (I think 4) and they are very hard to come by. Having all the "stuff" can increase the value more than the condition of the machine, as often they got separated from the machine and are very hard to find. I think this machine will not zigzag without a cam inserted.
I've got both an A and C versions, and one of them is in near prestine condition. Paid $35 for that one, it had belonged to someone's grandmother and they ended up with it and just wanted cash. Got real lucky that time.