Ric...
To get back to the original post question... Is the mower she found on CraigsList worth $500...
(That mower she said is a 1998 John Deere LT155).
I and others have agreed it definitely should be...
To further support that position... Here is a current, same year and model, used mower for sale at TractorHouse.com (respected source & they sell a LOT of used equipment)
1998 JOHN DEERE LT155 Riding Lawn Mowers For Sale At TractorHouse.com
Their price ... $1,595 for "USED 1998 JOHN DEERE LT155 38" DECK GOOD PAINT, SEAT, TIRES, STARTS AND RUNS GOOD"