In all honesty, for the Blade, I would keep it all stock. If you want to bling something out, then I would suggest upgrading to Align or the like. Being new to the game, learn how to fly without crashing, then start loading up on all of the bling. If you crash, and want to upgrade, then that is a good time to do it. but until then, stock is good. Most LHS's carry stock parts for this heli but they dont' carry the metal stuff.
As for eBay, there are good, and there are bad out there just like anything else. If you look at that sellers rating, he has over 22K (22,000, twenty two thousand lol, just had to stress this point) feedback points and a thumbs up rating of 99.9%. You just dont' get any better than that. being in a business, you are going to have members or customers that are just not happy. Some may just have a hot head and some may genuinely have an issue with something and leave a negative feedback without even trying to get it resolved. It happens, you just smile and walk away.
However this is where PayPal comes in REALLY handy. They have buyer protection on most all of their purchases. So if a member tries to screw you over, PayPal will do what they can to get your money back. Just like a credit card company.
So in short (too late huh? lmao), Ebay can be a great place to get great deals such as these knobs. Just look at a sellers feedback. If they are below about 88% or so, I will not buy from them. However if they are that low, I'm going to read the feedbacks and see what hte issue was. If it's something that has to do with shipping around the holidays, then it's not their fault. If it's with a specific item that I'm NOT looking at, I blow it off and order from them.
Hope this helps.