I've recently bought a bunch of cheapie markers from Ebay. My 9-yr old step-son and I took them apart to compare how they're built. Add a little lube and re-assemble. Test firing in the back yard is the best (I have an old department store mannaquin in the woods behind my house).
I usually don't dis-assemble the trigger assemblies -- I broke a VM68 all the way down to the atomic level -- I got everything back together without a problem -- except the damned trigger group. That took aaaaa long time