The short answer
Refusing outright can come across as cold, so accept a glass when you reasonably can. Tea is a real gesture of hospitality, not a sales trick, and accepting it in a shop does not oblige you to buy anything. If you truly can't, decline warmly and with thanks. Marrakchis are gracious about it.
In more detail
Mint tea is poured everywhere: in shop doorways in the souks, at riad check-in, in family homes and at the end of a meal. Sharing a glass is how a conversation starts, and it is often the most relaxed part of a day in the medina.
In the souks, a shopkeeper offering tea is opening a conversation. You can drink it, chat, look at the goods and still leave with nothing. Saying 'shukran' and moving on politely is completely normal.
Expect it sweet: sugar usually goes into the pot. You can ask for 'bla sukkar' (no sugar) or 'shwiya sukkar' (a little sugar), though traditionalists may look puzzled. In a home, the custom is three glasses, from a saying about life, love and death, and a soft 'baraka' (enough) after the third is understood everywhere.
Good to know
- Take the glass with your right hand.
- 'Shukran' (thank you) when you finish.
- Accepting tea is not agreeing to buy.