I feel like I've seen the reason somewhere before, but ... what is the function of "tane" in this sentence?
it is very commonly used after numbers, there is no translation in English
Are there any particular times when it would or would not be used?
It can always be used in places where you can count things with your finger, but can almost always be used. it is used a lot less with people, but it isn't wrong to use it
Is it improper NOT to use it? "Bu mahallede otuz satıcı var."
Exactly. It is totally fine to exclude it. The only time that it is actually required is when you are saying "I want X peices" where it would be the word for "pieces"
adelips: DL accepted this translation (Mar. 12, 2018).
Why not 'satıcılar?' 'Vendors' is plural.
The plural suffix is never used after numbers :)
"There are thirty vendors in this neighborhood."
Translation: Bu mahallede otuz tane satıcı var.
If the question was reversed then this is also correct:
There are thirty sellers in this neighbourhood.