You've said that the price is good, but that can kind of be open to interpretation, seeing that they go for close to a grand or more at used restaurant supply stores and is what folks are asking for them on Ebay. Can you give a rough ballpark of what they are asking ?
Berkel slicers are very reliable and have a great reputation. I have a 909E manual and a 919/1 automatic slicer. I love them. Hobart is the bigger name, but there's a reason why ITW Food Equipment Group, the company that owns Hobart bought Berkel.
Things to look out for. Definitely want to make sure that it powers up. Let it run for a few minutes to get warmed up. Listing for any sounds that seem kind of off, clanking, rubbing, grinding etc. Makes sure the indexing (thickness) knob turns and will move the gauge plate (big flat piece that the face of the meat rests on.) Make sure the product tray slides back and forth without catching and that the pusher moves/slides freely.
BTW, if you run into issues with some of the things mentioned above, it's not necessarily a deal breaker. Take the pusher. Say that it doesn't slide up and down the product plate very well. It could be that the pusher rod is bent, but even that's not a big deal as they are cheap to replace. But it could also be that someone used regular cooking oil or non stick spray like Pam on the rod to lubricate it, and it gunked the thing up. A simple detailed cleaning would fix that problem.
Also, check and see if it has the sharpener mechanism and the stones. The mechanism itself can cost $100 - $200 if you need a replacement. Stones are fairly cheap and readily available. If the person selling the Berkel knows anything about it, have them "demo" it, to include using the sharperner (if present). IF they don't know how, you basically just grab the knob on top, lift it up, rotate it and lower it so that there's a stone on each side of the blade. There's a level on the back (might have to flip it out) and you just push the lever and it should move the stones to engage the blade.