eBay dilemma...help!?

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So, let me explain:

- I bought a jacket off eBay a few days ago. At the time, it was only available in a size 10.

- Today, I was out shopping and saw the exact same jacket in a store - except it was a size 6. I tried it on out of curiosity, and it fitted well, but was kind of baggy.

- Now I'm thinking, okay, the 10 is going to be huge if the 6 was on the baggy side.

- So I come home, to find the seller has updated the listing and now stocks size 6! But there's only ONE size 6 left. I haven't received the size 10 yet. I expect it will arrive sometime this week.

Anyway, I know the 10 is going to be too big, and I really want that size 6 before it's gone. Would it be wrong in any way to go ahead and order the size 6, then return the 10 after it arrives? Would the seller feel like they were being messed about?
In my purchases section it says the 10 has been dispatched