D 21 tractor not moving?

I am looking at a D21. The vendor was straight up and said that he bought the tractor last year to resell, it runs good in his opinion, but, he also said he drove it in all gears last year and it was fine. He also pulled a disc with it to make sure it pulled under load. Says he pulls something with all his tractors for resale to make sure they function.This guy buys and sells tractors mostly for restoration and smaller farmers. He has a good reputation, and is actually distantly related to me we found out after talking. He said he is aggravated that this spring after the tractor sat inside for the winter, he started it up no problem, put it in gear and it won't move. He tried all gears and both ranges. He said ther is no noise of any kind from the trans or diffy. He really doesnt want to sel me the tractor and is not sure what he is going to do with it. He says there is no money left for him once he starts pulling tractors apart. He also said its far to nice to part out, and couldnt bear to do that. My question is what causes a D 21 to move fine upon storage and then...not? I have several ideas but want to hear from members here. Sorry for the long post but this tractor would be a silly dream come true for this country boy living in the City, thanks, Trev.

With the clutch pedal "up", try and shift the 4-speed transmssion to see if there is any rotation or grinding.......then go on to the range lever and see what it does. I suppose the clutch disc could have the center ripped loose on it.

Went to pick up a D14 to do a clutch job roe a guy becuz it would not release -- when driveing it up on trailer something let go and then it would not move, what happened was the center came loose as the DR. said. At first a couple rivets came out that got bound up in the lining to flywhee area and then all the rest came out of the center and tractor would not move.

On my 21 it worked fine in the fall , sat all winter and spring then when summer came and i had some time to play it would slip under a load. I ordered a new clutch and pressure plate so i'd be ready when i split the old gal and all i would need to do was get the flywheel turned. The shop freed up so i ran her in and split it, the disc had some wear but not that much for the way it was acting and there was some hot spots on the flywheel which i expected. Turned the flywheel put new seals and clutch and pressure plate in and slid it back together. When i tried adjusting the free travel there just was not enough travel of the parts. Split it again and began measuring everything. I was ready to chuck the throw out bearing holder in the lathe to gain some clearance when i realized how the bell crank clutch linkage worked. On top of the bellhousing there is a pocket that the linkage pivots in and I ended up chiseling and blowing out 45 years worth of dirt, acorns, grass, corn cobs, sticks and whatever elses critters had placed in there. After that i had a whole bunch of adjustment room, probably could have done that to start with and not spent hard earned dead presidents on new parts. long story short a few winter stored acorns by some ground squirrels could make you a deal on one of the best looking tractors A-c ever made.