If you buy used kegs from any other source than distributors or breweries, they may be ill gotten ... don't sweat it.
I picked up mine off CL for$50 a piece. The guy had bought them off CL planning to convert to keggles, himself.
Until retired kegs are permanently marked, you'll never really know what you are buying.
Get them and clean them up. Enjoy your brew day...
"Make sure that the beer - four pints a week - goes to the troops under fire before any of the parties in the rear get a drop." -Winston Churchill to his Secretary of War, 1944