It seems pretty clear after reading the poem “Goblin Market” by Christina Rossetti, that it’s theme has to do with some sort of addiction. The poem uses fruit as the symbol for the addiction.
Some suggest that it is sexual addiction, which is possible given the way the goblins treat Lizzie when she goes to get fruit for her sister. It also sounds like it is some other sort of addiction like a drug because the poem mentions that another friend died from it, and it talks about Laura being listless around the house, which sounds like withdrawal of some sort.
Most likely, the poem is talking about addiction in general, and references multiple sorts of addiction.