I paid 100$ for the 707; the motor brush were worn (it cost me 25$ to replace it) and the machine was coming with no other accessories than the case and the sewing table. It was the first Bernina I have found in my area after looking for months, so I was glad to have it for this price. Afterwards, I don't think it was a very good deal.
I found the 831 a month or two after; paid 150$. The 831 only have the "utility stiches" (not decorative) so it is worth less than the 830. Still, it`s the same body and the same fine sewing machine. It was in absolute perfect condition and coming with a tons of feet (they are worth 20$ each). So this one was a good deal.
A few months later, I found the 730. I paid 75$ for it with a cabinet (didn't have the place, so I give it to charity). It was coming with the case, all the original accessories, and even more feet than the 831 (they use the same) but no table. The capacitor was leaking on this one; I had to remove it. There was also an issue with the knee controller. I was able to fix everything quite easily, and the feet alone were worth more than what I paid for, so this one was also a good deal IMO.
So, is this 530 a good deal for 100$? Only if it is coming with a lots a feet, and if, after trying it, you found it is sewing smoothly, with no weird sound coming from the motor (you have to make it run for a few minutes). You have also to consider that it is knee operated (like my 730). Some like it, some don't. I like it, and it`s useful for me because I have small children so they can't mess with the foot pedal of this one :-)
If not, try to offer 50$... or wait for another one.