When am I supposed to say tion, kion versus just kion?
Mi ŝatas tion, kion vi aĉetis.
Mi ŝatas kion vi aĉetis.
I've come upon a number of hard and fast answers to questions like this over the years and I think most of them are wrong. The sentences you give are essentially the same in meaning. If you think it's better without "tion" you can drop it.
I can't find the answer that I just wrote (within the last week or two) on this same question, but in short, you can generally drop the ti-correlative in sentences like this, unless there is a preposition or other function word involved.
See PMEG for additional detail.
From the direct English translation (which is of course not always a reliable guide):
In the second case it's a non-obligatory relative clause, providing further information about what the speaker likes. The first case is a comment about someone's shopping.
Possible contexts:
Note: I still see myself as a komencanto, so this explanation is mainly based on a linguistic analysis. If this is counter to common Esperanto usage, please correct me!
There is a lot of research going on on Information Density and Linguistic Encoding, therefore a final answer is not (yet) possible. The first encoding tion, kion is less dense than the second one kion; therefore it is preferable in cases when misunderstandings can happen easily (noisy environment, the rest of the sentence is complex and/or surprising). In all other cases, we prefer the more dense encoding: it allows us to get the message across more quickly.