Hmm, I learned it differently: Zhe ge shi shei mai de? But I could be wrong.

yes, you can. its a correct sentence. btw, i think "shei mai le zhe ge" is better, though "shui" is not bad. :P

It's not incorrect. But you'd better say "sheí mǎi le zhè gè " Or just as Eliot wrote "zhè gè shì sheí mǎi de" Both of them are correct. :D

you got it. but for the sujected about the usages of "le" all the pasttenses forms, doesn't use "le" there is no such certain rules to mastered it. hope you have fun, lil by lil.

yes you are right, you can say"谁买了这个?"（“shui mai le zhe ge?”）