Look for the slide plates, they're not hard to replace if they're missing, but the new ones do look different. Make sure that you take some fabric and stitch with the machine, even if the belt is missing, use the handwheel to see what kind of stitches you get.
It looks like this machine has been electrified...the treadle pedal is missing. Not hard to replace, you can pick them up on ebay, but if you're looking for a treadle, it's going to effect the total cost of the machine. The cabinet looks pretty, love that the manual is there and some of the attachments.
I'd probably buy it, but would use the missing treadle pedal as a negotiation point, along with and problems the machine itself has. I'd offer $40....the pedal is going to cost you to get it shipped....they're cast iron and heavy.
ETA: Looking closer, looks like the flywheel is gone too...if you're looking for a treadle machine, keep looking unless he'll let you have this one SUPER cheap...