「買ったやつ」the guy who bought, or the thing that was bought?

Question

I am reading the Japanese translation of Matilda by Roald Dahl. Spoilers ahead.

Mr. Wormwood has just finished explaining how he cheats his customers at his second hand car store. He fills the engine with hay to get rubbish cars to work again!

Matilda (his daughter) then asks him:

どのくらい長く走るの、またガタガタしはじめるまでに

to which he replies:

買ったやつがかなり遠くへ行ってしまうまで走れるんだ

I interpreted 買ったやつ to mean "the guy who bought the car". Mr. Wormwood is rather impolite and would have no problem referring to his customers as such.

However, others have cast doubt on this interpretation. An opportunity to learn presents itself! Which is more likely to be correct here?

After this debacle I looked up the original English, here it is:

"How long will it run like that before it starts rattling again?" [Matilda asked him.]
"Long enough for the buyer to get a good distance away,"

Answer 1

This 買ったやつ is "the person who bought it", so your initial interpretation is correct.

(その車は)[買ったやつがかなり遠くへ行ってしまうまで]走れるんだ。

The car can run [until the 買ったやつ goes quite far away].

The implied topic of the sentence (the subject of 走れる) is obviously the car, and this sentence specifies a different subject in its subordinate clause (the まで-clause) using が. This means 買ったやつ refers to something other than the car itself. If it referred to the car, you wouldn't have to specify the subject here.

(その車は)かなり遠くへ行ってしまうまで走れるんだ。

The car can run until it goes quite far away.