That depends on what you can get at a reasonable price from the wholesaler or what you can propagate yourself. Winter bedding plants are what seems to be selling in our market.

Pansies , any herbs for example garden mint , thyme , rosemary , lavendar , basil ,sage , chives.
Even if you bought the herbs out of a garden centre you can always slap a bit more on the price.
Miniature Roses.I often buy these out of somewhere like Tesco's or morrisons and split them up, because you get about 4 to a pot .
Primula's. Even put bulbs into a pot of compost and sell them that way , you'll have a good variety that way.Daffodils tulips and even hyacinths

Bare rooted shrubs? Evergreens?