Learn Turkish: Ben satıcıdan poşet alıyorum. - I am getting a bag from the seller.

The word for seller is satıcı. To say "from the seller", we add the ablative suffix -dan. Because the last vowel in satıcı is ı (a back vowel), we use -dan instead of -den. Also, since the word ends in a vowel rather than a hard consonant, the suffix starts with a "d" rather than a "t".

Why is there no word for "a" before poşet?

In Turkish, a plain noun placed directly before the verb often represents a general or indefinite object. While you could say bir poşet, leaving it as bare poşet perfectly translates to "a bag" here. If you were talking about a specific bag ("the bag"), you would need to use the accusative case and say poşeti.

Does alıyorum mean "getting", "taking", or "buying"?

It can mean all three! The root verb is almak. It is a very common verb that translates to taking, receiving, getting, or buying depending on the situation. Here, getting a bag from a seller covers all of those meanings.

Where does the word satıcı come from?

It comes from the verb satmak, which means "to sell". The suffix -ıcı or -ici is attached to verbs to describe a person who does that action, just like the "-er" suffix in English. So satıcı literally means "seller".