間 and 隙間: a position between things and a physical gap
Compare 間 read あいだ with 隙間, distinguishing a position between two references from an unfilled gap.
A location between two references
間, read あいだ, identifies a position between two things. 郵便局と銀行の間に薬局があります。 places a pharmacy between the post office and the bank.
In AとBの間, と joins the two location references. This does not turn into the containment relation in 薬局は銀行の中にあります。
A gap between objects
隙間, read すきま, means a physical gap. 窓の隙間から風が入ってきます。 says that wind is coming in through a gap in the window.
棚と壁の隙間に物が落ちました。 says that an object fell into the gap between the shelf and the wall. Both objects identify the gap.
Being between things does not imply vacant land
建物の間に店があります。 locates a shop between buildings. 間 gives that relation rather than necessarily naming an empty piece of land.
隙間をふさぎました。 says that a gap was closed off. Its size and the material used require separate information.
Does 二つの建物の間に店があります。 describe vacant space between the buildings?
Show the answer and why
No. It uses the buildings as references for the shop and explicitly says that a shop is between them.