Learn Turkish: Kırmızı güllü buketi satamam. - I cannot sell the bouquet with red roses.

The suffix -lI (which becomes -lü here because of vowel harmony with the ü in gül) turns a noun into an adjective meaning 'having', 'containing', or 'characterized by'. So gül (rose) becomes güllü (having roses). Together, kırmızı güllü buket means a bouquet containing red roses.

Could I use ile or -le to say 'with red roses' instead of the -lü suffix?

No, they serve different purposes. The word ile (or its suffix form -la / -le) is used for accompaniment (like going somewhere with a friend) or an instrument (like cutting with a knife). The -lI suffix is used to describe what a noun is characterized by. Since the bouquet physically contains red roses as an attribute, you must use güllü.

Why is there an -i at the end of buketi?

That is the specific accusative case suffix. Because the speaker is talking about a specific bouquet ('the' bouquet with red roses) that is the direct object of the verb to sell, buket takes the -i ending.

I know satmam means 'I do not sell'. What does the extra -a- do in satamam?