How about freecycle? You're not allowed to ask for anything in return (money or traded goods) but this is a great way to rehome that which you would throw away without it going into a landfill... See if they have an active community in your area too... it might even work for the encyclopedias... though the supply and demand situation on those buggers is working against you...

The Tony Little Gazelle might do. The Supra (Sears sells it) has a weight capacity of 325. No knee bending which is why I bought one. I have some minor knee injuries and arthritis which cause periodic painful knee bending. What about a rowing machine? Can he bend his knees with no pain? I have a friend who recently had a knee replacement. She was referred for gastric bypass surgery which she decided not to do and lost some weight on her own, about 45 lbs. They wanted her to lose 100 prior to surgery but went ahead with it anyway after the 45 pound lose. She has continued to lose weight as she recovers and starts working out with a personal trainer this week.

On cork flooring, an area rug might just do the trick.

Sure....area rugs with natural backing. In other words you will AVOID anything that contains RUBBER or LATEX or VINYL. A rectangle of carpet that is cut and bound will do the trick. I would NOT expect to get anything "squishy" underneath the treadmill. The "squish" comes from rubber/latex/vinyl. Go with an area rug that has a backing that is safe for vinyl and you will be just fine.