I have a 2008 Toro z5000 and the stamped mower deck has rusted out beyond repair.  I have located a fabricated 50 inch deck from a newer Toro MX5050 in good condition about 200 miles away.  From photos they look interchangeable and from what I can find out both use the same 1/2 x 141 inch belt.  The MX5050 deck is much heavier with different spindles.  Is this a possible bolt on swap ???????   

Looking at the parts diagram they appear to be the same:
 https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/toro/riding-products/74370-timecutter-z5000-riding-mower-2008-sn-280000001-280999999/50-inch-deck-assembly-no-110-6790
 
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/toro/riding-products/74770-timecutter-mx-5050-riding-mower-2016-sn-316000001-316999999/50-inch-deck-assembly 
 
Look closer at the pic's and compare to yours...
 
.

Wish the diagrams had measurements but just from looking it appears the rear mounting brackets may be spaced differently and the Z5000 brackets are further back.  The front support bracket is also different, higher and further back on the Z5000.

For those potential differences, you can have the seller take some measurements,
give them to you to confirm yea or nay..
 
But it sounds like you have your answer...  :(
 
 
.