"Shopping" is the process of looking for things that you may or may not buy.
If you definitely plan on buying a ladder and you're pretty sure the store will have a ladder that you want to buy, it's fine to say "I'm buying a ladder." If you just want to see what kinds of ladders are available, then you can say "I'm shopping for a ladder." If you see one you like, you can buy it - "I went shopping for a ladder and I bought one."
One can say "I'm buying a ladder" or "I'm shopping for a ladder," but you couldn't interchange the participles in these sentences. "Buying" probably has more of a sense of immediacy—but that's disputable. "Buy" also has many meanings not at all related to "shop" and is part of many phrasal verbs, some with unexpected meanings. The only phrasal verb I can think of with "shop" is "shop around," which has the sense of Miss Julie's phrase, "looking at items without buying them" and can be applied to potential friends as well as to goods.