$800 is way to much for a CB125 unless it's mint, mine is in perfect running condition, repainted, rebuilt front forks, good seat, new tires, o-ring chain, and I would sell it for $500, I've also owned a cb100 and still own a cl100 (scrambler) and all 3 basicly have the same engine, only the 125 is a larger bore, but other then that they are the same bike, and it's a solid engine, and on the off chance that you do burn it up, they are cheap to rebuild ($100 to bore it out and replace pistion and rings) it was an already abused engine and I did have nearly 400 pounds of passenger and rider on a 200 pound bike, I love my cb125, and it gets an extreamly reliable 68mpg, and tops out at 55mph (hitting the 9,500 red line) or if I duck down it will go 60mph (10,000rpm) on a flat flat road, and it's such a simple motorcycle that I don't think it will ever leave me stranded, it is however slower then most people would want.
if I get another bike any time soon it will either be another moped, or a Ninja 250.
I never owned a Rebel but I put 30,000 miles on a CB125. I think I paid $597 for it brand new. I once put 4,000 miles on the 125 in 8 days. I was going to ride to Alaska in I think it was 1978. It rained a lot that year. 100 miles of mud is hard on a chain. I turned around and came back home. Around 16,000 miles the piston broke. The crack ran from the piston pin to the crown of the piston. When the crack doubled back a chunk of the piston head bent down and it left me stranded. No cylinder damage but I bored it out a lot anyway. I gained enough torque to run one tooth larger on the engine. That led to the speeding ticket in Oregon. I had a tailwind in the Columbia gorge and got a ticket for 78 mph. I bolted a bar across the bottom of the tirple clamp. I locked the throttle wide open, laid back on the luggage and steered with my feet. I am not recommending anything, only admitting. I got right at 65 mpg no matter whether in town or on the freeway. Obviously I liked the little bugger. I moved up to 500 four Honda and gave the 125 to a nephew. It was running perfect when delivered but he destroyed it in short order.
One little bit of advice. 8 days at about 7,500 rpm can make your butt hurt really bad. The numbness seemed to last about 6 weeks.