I am trying to find out the year that this tractor was purchased/sold?

Sorry but I could not find the date of manufacture but many aprts are no longer available so its been awhile. http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Riding-mower-tractor-Parts/Model-917273050/0247/1509200/P9010345/00005?blt=06&prst=0&shdMod=917273050 Why do you ask?

We purchased a new tractor and want to sell the old one and wondered how much it might be worth. It is in excellent condition, however the pulley's on the mower deck need replaced.

"pulley's on the mower deck need replaced.
" that means not so great condition. Unklike cars folks don't frequently search for the year of manufacture when buying a used tractor. On sale many Sears tractors go for about $1100 new depending of the HP. Search for a new one similar to yours taking into account HP and mower width, price it at 1/3 to 1/4 of that and see if you get a bite.