Is there a "quality" difference between certain supplements?

I know with some vitamins there is a difference (vitamin C & ester C, Calcium and Coral Calcium) but what about supplements used for weight lifting / general fitness?

I am currently taking COEnzyme Q-10, L-Carnitine, Green Tea Extract, Flax Seeds, Fish Body Oils, Alpha-Lipoic Acid, Gingko Biloba, a multivitamin, and, finally, vitamins that help eye sight.

Practically all this is coming from GNC. It's really a budget cruncher. However, I'm afraid to buy something from Walgreens because I don't if it's of same quality. Call it a marketing gimmick, but I trust GNC.

Anyway, Walgreens is having a buy-1-get-1 free sale and I'm really starting to think about this.

So, I want your advice... is there a difference between brands when it comes to, say, L-Carnitine?

Let me know. Thank you!

Honestly, I'm not sure if GNC has good quality, but you can always choose for yourself.

Huge chances are that supplements sold at a supermarket are made synthetically. That's the problem with a lot of vitamins nowadays. They're made from cheap synthetic materials and chemicals that aren't very good for you. You want supplements made from natural sources.

I'd go to a health food store and buy there. Or, if that's too much money, buy from NOW or Nature's Way on eBay. Those are companies that make their supplements from all-natural sources and are often sold at health food stores. I suggest those because they're sold often on eBay.