What's the impedance and sensitivity of those speakers?
As Solar Flares said, the B60 is basically a combination of the BP6 preamp and 2BLP power amp in one box and as such, a splendid little amplifier. A true high-end piece. But it is not happy driving low impedance loads!
Mind you, neither is the B100SST exactly a powerhouse and it too is happy with a high(er) impedance but it is more powerful than the B60 nonetheless.
Also, be sure to get really familiar with what has actually been done to the amplifiers. Even if the cosmetics are good, that doesn't mean the amplifier did not have a busy life. Request additional information on who carried out the service, when and what was actually done. There has to be some trace of it - a receipt or other documentation. Remember that when buying used, this can quickly turn into a nightmare. Personally, I would much rather have a mint entry level amplifier than a top-end one that's had a busy life. Not even a hard life, just a busy life.
Now, Bryston's reputation is excellent but that doesn't mean you have to buy a proverbial lemon, especially in the case of B100SST. It's based on the BP16 preamp and so all the controls are electronic and sometimes they can misbehave. Make sure to do a thorough checkup of all of its features. Being a Bryston, I can't imagine it's that cheap and given the fact you're buying used, chances are, you work for your money like most of us.
Cheers - Antun