Learn Hardcore Ukrainian: Ми йдемо на розпродаж, аби купити капелюх за хорошою ціною. - We are going to the sale in order to buy a hat at a good price.

Йдемо = we’re going on foot (from йти “to walk/go on foot”).
Їдемо = we’re going by vehicle (from їхати “to go/drive/ride”). So your sentence explicitly means walking.

Can I write ідемо instead of йдемо?

Yes. Йти/іти are accepted variants. In modern standard Ukrainian, й- is more common in writing: йду, йдеш, йдемо. You will also hear іду, ідемо in some regions (especially western Ukraine).

Why is it на розпродаж and not в розпродаж?

Ukrainian often uses на with events/activities and institutions: на концерт, на виставку, на роботу. A розпродаж is an event, so на is natural. Use в/у mainly for going into physical spaces (e.g., в магазин).

Why is розпродаж in the accusative case here?

After на with motion (direction), you use the accusative. For inanimate masculine nouns like розпродаж, nominative and accusative look the same in singular. If you were already at the sale (location), you’d use the locative: на розпродажі.

What’s the difference between на розпродаж and на розпродажі?