The most common aquarium drifwood kinds are Manila and Java.
If you buy those you should be safe.
Personally I have all manila pieces in my tank and I like them a lot, bought them fairly cheaply in a local store.
No, that's just a regular piece of old wood. I wouldn't buy it. Besides, it's not cheap either.
I did a quick search on ebay and there doesn't seem to be any good driftwood piece.
You should look into aquarium commercial websites if you don't have the chance to go to a regular store.
Oh, and another popular kind is Mopane.
I have Mopani in a tank that's not filled with water, and it works well. But, if you're looking for something to use as a basking area, consider corkbark.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-