[X4U] readying iMacs for sale

[Mary C.Youra]

My two college age daughters recently decided to replace their Indigo 
iMac 500s (Summer 2001 models, I think, though not important for 
purposes of my question) with  14" iBooks. They'd like to sell the 
iMacs. The iMacs came with disks for OS 9.2.2 and 10.1..5, but have 
been upgraded to 10.3.5. What should be done to get them ready for 
sale? What is customary, and what is necessary from a security 
standpoint?

Thanks for any advice.

Mary