Lilys are coveted bags in the Coach world...if you trust that the bag is in good condition, feel that the bag is worth the asking price, and you can afford it (I wouldn't skip the mortgage payment to buy a bag, KWIM?), then I say GO FOR IT! You just don't know when another one will pop up. I LOVE the Lily, but I just don't think the straps are long enough for me...I'm a shoulder bag kinda girl.
You might want to ask the seller if it's her own bag and/or where she got it from, depending on the answer. I looked at the photos, looks pretty good. I don't like the curling of the front straps, but it seems to me on all the metallic Lilys I see, most of them do that.