The Klingon forehead arc is great stuff. We already had the problem of differing foreheads, the explanation is excellent.
Ebay is perfectly safe if you pay attention. I've been using it since 98, have over 600 feedback and have bought practically everything that way. I have been ripped off once, to the tune of 12.00. If you buy stuff via paypal you can get your money back anyway. Anyone with a "is this okay" Trek question should post it on the board, you will get told quickly if it is genuine.
Oh and like KingDaniels says, you will save heaps of money. Amazon can be very over priced.
A great place to buy dvd's is:
Very cheap postage (and I'm in Australia), great range, often mind blowingly cheap for tv. I bought all my Babylon 5 extra stuff from them (the rangers etc..) For obscure things that cost a lot in Aus this is where I go.
Here's their Enterprise page:
I bought some Stargate seasons from them for something like 8 dollars! They often reduce one season in a series. They might not be your cheapest for ENT but I highly recommend always checking with them, an excellent company all round.