Tender Lizzie could not bear To watch her sister's cankerous care Yet not to share. She night and morning Caught the goblins' cry: "Come buy our orchard fruits, Come buy, come buy;" - Beside the brook, along the glen, She heard the tramp of goblin men, The yoke and stir Poor Laura could not hear; Long'd to buy fruit to comfort her, But fear'd to pay too dear.
Lizzie hates to see Laura wasting away, and not be able to do something.
It's also driving her bonkers that Laura won't tell her what's wrong. Lizzie can tell that something is wrong, of course, but "not to share" her sister's pain is hard for her.
She can still hear the goblin men, even though Laura can't.
They're still out there, shouting "come buy, come buy," every "morning and evening."
Lizzie wants to go buy her sister some fruit, but is afraid of "pay[ing] too dear."
"Dear" is a British-English word for "expensive," so Lizzie's afraid of paying too high a price, either literally, in terms of the money the fruit would cost, or figuratively, in terms of the "cost" to her own health and happiness.
It's not made totally clear, but it seems like Lizzie's probably more worried about the figurative cost to her health.
She thought of Jeanie in her grave, Who should have been a bride; But who for joys brides hope to have Fell sick and died In her gay prime, In earliest winter time With the first glazing rime, With the first snow-fall of crisp winter time.
Lizzie can't forget what happened to Jeanie, the other young woman who tried eating the goblin fruit and ended up dead.
She thinks that Jeanie would have been married by now.
But she couldn't wait for the "joys brides hope to have," and ate the goblin fruit and died "in her gay prime."
The "joys brides hope to have" probably refer to sex. (This is about as explicit as Rossetti gets, and it's not very explicit.)
Jeanie died in the winter, around the time of the first "glazing rime," which is a kind of hard, dense frost, and around the time of the "first snow-fall."