How do I find the coral claw?

Why can't the girl sell it to me?

Hmmm. That's odd. Generally, you walk into Winterhold, see Birna and Ranmir arguing, go into the shop, and the convo progresses to where you get the claw. If that's not happening.... I dunno.

This walk through of that quest includes plenty of info about getting the claw. Note there are two places you could get the claw. http://en.uesp.net/wiki/Skyrim:Yngol%27s_Barrow

Hope the info helps.

Also my game freezes and can you start the quest over agin on Xbox 360 skyrim?

Starting the quest over would required loading a save before you accepted the quest. Best to save right before entering into a conversation with a quest giver, that is if you know they are, if not save often so you never have to go back to far in game play.

I'm curious about something. Are you trying to buy it from her store inventory? The claw has its own conversation topic about the "bad deal," which you get to by asking how business is.