Learn Spanish (Latin America): ¡La de postales que tiene ese vendedor!

In this exclamatory structure, la de actually stands for la cantidad de (the amount of). Because cantidad is a singular feminine noun, you always use the singular la, regardless of whether the noun that follows it like postales is singular or plural, masculine or feminine.

Could I use lo de instead of la de here to mean the same thing?

No, you cannot. If you said lo de postales, it would completely change the meaning to 'the matter of the postcards' or 'the thing about the postcards'. To exclaim about an extreme quantity of a noun, you must strictly use la de.

Why is the subject ese vendedor placed at the very end of the sentence?

In Spanish exclamations and relative clauses introduced by que, it is very common and natural to put the verb tiene before the subject ese vendedor. You could technically put the subject before the verb, but inverting the order to put the subject at the end sounds much more idiomatic to native speakers in this structure.

Since this sentence expresses strong emotion, why is the verb tiene in the indicative rather than the subjunctive?