Learn Spanish (Latin America): Él planeaba venderlos en la reventa cuando el precio subiera. - He planned to sell them on resale when the price went up.

This is the core of temporal backshifting. If this sentence were in the present ("He plans to sell them when the price goes up"), we would use the present subjunctive for an anticipated future event: cuando el precio suba. Because the main action is in the past (planeaba), that anticipated future event must "backshift" into the past too, becoming the imperfect subjunctive: cuando el precio subiera.

Why is planeaba in the imperfect tense rather than the preterite (planeó)?

The imperfect is used here to describe an ongoing mindset or background state in the past. He "was planning" or "had the intention" to sell them. If we used the preterite (planeó), it would sound like a single, completed action of drawing up a plan, which feels less natural when describing a lingering intention dependent on an uncertain future condition.

Could I move the pronoun los to the front of the sentence?

Yes, you can say Él los planeaba vender... instead of Él planeaba venderlos.... When you have a conjugated verb followed by an infinitive (like planeaba vender), object pronouns can either go right before the conjugated verb or attach directly to the end of the infinitive. Both are perfectly natural and mean the exact same thing.

Is en la reventa the standard way to talk about the secondary market or scalping?