What is the word for "they/them"?
The translation of "they" depends on what it refers to.


Use "他们" if you are talking about a group of people.
Use "她们" if you are talking about a group of females. Note that if you are certain that they are a group of females you should always use "她们" instead of "他们".
In comparison, if there is at least one male in the group of people, or if you don't know about their gender, you should use "他们".
As for "祂" I have only seen it when Christian people refer to God. There's only one God so you don't want to say "祂们". For other gods, we usually use "他" or "她", depending on if that is a male god or a female god.
"牠" is for animals. However it is seldom seen in modern CHS (Chinese Simplified). Now we use "它" for animals too. It is considered a character in CHT (Chinese Traditional).
"它" is for objects, including animals.




For your example:

You should use "他们" in your example sentence instead of "他们" because actually you are talking about the people in the shop.

No, they only sell x. => 不，他们只卖 X。



Some other special cases you may want to know:


When we assimilate some object to a person (拟人, personification), we use the corresponding "他" or "她" instead of "它". Therefore you may also use "他们" or "她们" in this cases.
For pets, Chinese people don't think they are people - No they are never people. You should always use "它们" for your pets as you do for other animals. Exception happens if you are following point 6.