I honestly don’t know of any “bad” thermal paste, and one gram will be much much more than enough. Keep in mind that the layer of thermal paste between the chip and the heat sink will be less than paper thin, so when you apply it to the new chip, only use a drop. It will spread out to fill the space. The instructions that come with the thermal paste should explain the procedure. I suggest that you take a look at some YouTube videos regarding the procedure before you do it yourself, just so you got good idea of how much to use.

Edit: just as a sidenote, I have never, in all of the years eBay has existed, I felt comfortable buying anything from there. But that’s just me, personally. You can get thermal paste on a lot of places (it’s cheap), and I would recommend buying it direct from a quality retailer.