Easiest way might be to call
Best Buy first. From Best Buy's website: Call 1-888-BEST BUY (1-888-237-8289) for a BestBuy.com price match request. Please be prepared to direct our representative to the ad or website that is displaying the current lower price you would like to match.
http://www.bestbuy.com/site/apple-w...1e6b6b8964bf99121d90INT&id=pcmcat748300489188
You are still within your return and exchange window (15 days), so you can request a price match against Best Buy's own lower price. They will ask you for your order number (or eBay store equivalent).
Worst case scenario, the CS rep will just recommend you go to a store to return or exchange your watch for a new one at the lower price. I know that can be a hassle backing up your watch, resetting it to factory settings, repackaging everything and going to a store, but Best Buy doesn't charge a restocking fee.