Honestly, they are all about the same. The one I used (HappyBuyItNow) is no longer on ebay from what I have seen. They actually sent me an e92 trunk spoiler by mistake and replaced it with an e90, never asking for me to return the first one sent. Most of the companies are really solid...just do your homework, send messages to people who have purchased the same parts (contact people having left feedback for the seller), and you should be fine.
As a reference, I got the spoiler and splitters for around $300 - $350 shipped...cant recall exactly. A bit better than the OEM pricing, eh? And I sold the e92 one for $180 on an eBay auction. Good times!
2010 e90 M3