Not to buy anything from a retailer that doesn't stock spares/consumables.

Went to buy a pair of shoes earlier this year. Salesman had racked the
shoes up, and I asked for 2 spare pairs of laces, only to be told they
didn't stock laces. Just walked out.

Looking for a new bathroom bin, noticed a 3L jobbie in Tescos who - guess
what - don't sell 3L bags to fit. No sale.