On the whole I personally think that the chance to buy near new products at vastly reduced prices is a good thing, but I would say that because we sell the very same. Here I will try to be objective and informative.
There are many reasons why good stock from High St retailers becomes graded stock. Some of these are...
Surplus stock and over orders. These normally end up on ebay as new, which is fair enough, because they are new, but, they should be un-opened untill they reach the ebay buyer.
Ex-display. Sounds good on the surface. What you need to do is imagine you are walking around a store, look at all the items that are switched on, and which are not. Tv's are all on for eight or so hours a day, but televisions can last for years. So maybe this is a good test run before you own. Also no one touches those bigs lcd tv's either. Cameras don't get touched or swiched on till a customer wants a demo. Laptops! I have never seen a child (incl my own) go past a laptop without banging the keyboard.
Customer Returns. Some un-named High St retailers will give a no questions asked return policy. And customers take advantage of this. Some do legitamatly via unwanted gifts etc. Some though will buy, lets say a sat nav, go on holiday with it, and return it to the store when they get home. Some customer returns are for faults or missing accesories/parts. This is where the Ebay shop that is selling should test all products where possible. All items are normally test prior to re-ditribution, but some do slip through the net.
Damaged Box/packaging. High st retailers wont sell something that has a damaged box. But Ebayers don't mind a damaged box if they are goning to bag a bargain. Ebay shops should be inspecting any damage to goods. Quite often we see a box thats got a big crush or dent and the item inside is fine.
Grading system can vary from seller to seller. ie. Shop X may call a paticular radio A- Shop Y may call the same radio B-. The only way you as a customer of these ebay shops is to be confident of a good deal is to check out their overall feedback.
So in a nutshell, if you want a good bargain but don't want to pay high st prices, and don't mind a mark or a scratch here and there, graded is for you. Don't forget, it wont be new, but also, it wont be well used.