Present tense ¿Qué compras? = What are you buying?
Past tense ¿Qué comparaste? = What did you buy?
You're thinking of "what do you buy" and yes you are correct, but it could also translate to "what are you buying" from what I understood from previous lessons, like for example "yo como" could mean "I eat" and also "I am eating", I don't know the grammatical terms :'D but you could understand it from context I guess, like this sentence shouldn't translate to "what do you buy for the new house" so you know it's "what are you buying.."
Jr0007 and ChrisKing, in Spanish, the form of verb using the estar + -ndo endings is reserved for what one is doing at the moment. (I am typing NOW; as I recall, it's called "present progressive.")
So if you actually saw him/her in the furniture store, preparing to buy a couch or lamp (or whatever) for their new house, your conjugation is correct.
But you may be in a coffee shop, at school, or at their old house before they move to the new house, talking about what they plan to buy for their new house, and Spanish uses the simple present tense to express that time frame, a "near future" generalization. So its meaning is, "What (kinds of things) are you (going to be) buying for your new house?"
I hope that helps you clarify when to use the different verb tenses. That's the understanding I have; if I erred in any way, advanced learners please correct me. :-)
I don't agree with you. "What did you buy for the new house" is a question that asks what thing(s) you bought for in the new house, for example furniture, or lamps, curtains, etc. This is what is meant in the Spanish question. "What did you buy the house for"? (with "for" at the end of the question) has a totally different meaning, or even two different meanings. 1/ How much did you pay for the house? 2/ Why did you buy the house?