Learn Finnish: Ne myi koko pelihallin kahdella tonnilla!

This demonstrates the adessive of price. When you state the exact amount something is bought, sold, or rented for, Finnish uses the adessive case. It acts exactly like the English word "for" in phrases like "for two grand".

Why is the verb myi in the singular when the subject Ne means "they"?

In colloquial spoken Finnish, the third-person plural pronoun ne (used for "they" instead of the formal he) is almost always paired with the third-person singular form of the verb. In standard written Finnish, this sentence would begin with He myivät.

Does tonnilla refer to a physical weight of one ton?

Not in this context. While tonni literally means a ton, in everyday spoken Finnish it is very common slang for a thousand (a "grand"). Here, kahdella tonnilla means "for two thousand euros".

Why is pelihallin in the genitive-accusative case? Doesn't the word koko change how it behaves?

The word koko (whole/entire) is an indeclinable modifier, meaning it never takes any case endings itself. The noun following it simply takes the case it would normally need in the sentence. Because the arcade was completely sold, it acts as a total object and takes the standard genitive-looking accusative ending -n.