There are GC re-sale sites, but you won't get 100% of the GC's value.
The Christmas gifts sound like a good idea too, but one of the best parts of ordering a gift through amazon is shipping it to the person for free (not sure your loved ones want to actually receive their Christmas gift in July?) If your GCs don't expire, though, you could save them for November / December. Amazon had some awesome sales last year.
I agree with you about their household / grocery prices not being very good.