I won an item for £10.50... but seller will relist if I don't give her £45??

leave her a negative. it will hurt her more than it'll hurt you since she has so little feedback. ESP if she's a seller, people will actually think twice before buying/bidding from her also.
 
She may leave you retaliation feedback. But I think ebay just changed their policies effective 2/1 that buyers couldn't receive neg. feedback.

I didn't write to her as it seems like it wouldn't do any good (She already relisted! And gave me barely any time to reply before she did). But I left her this NEG:

Non performing seller. Won for £10.50 but told me to pay £45 or she wouldnt sell

She now has the enviable feedback score of 33% :rolleyes: I also reported her using those links Lee69 provided. Hopefully she will get kicked off ebay soon!
 
WTF

I saw a person currently selling a large Coach Lily who states they won't sell the item for less than 850 but there is no reserve. I wanted to report to eBay, but didn't.

You should absolutely report in this case. I personally would have sent the money I owed and demanded she send the item. Then if she didn't forward a copy of the refund and email to ebay.