I can tell you right now, adopting is probably a lot easier than raising such a fragile creature as an atricial baby bird. At the very least, you should get an adult. Just be sure you look into their proper housing and how long they live so you know what you're getting into- are you still going to be equipped to care for a cockatiel in 20+ years?
If there was snot coming out of his nostrils and he was less active, he was sick. Baby animals can take a turn for the worse very, very fast, and it's possible he was in bad condition already.
Again, be very suspicious of any place that's selling pre-weaned animals of any sort. Always ask yourself, and them, why, and carefully consider the answer.