Is this a good saxophone?

Since they're advertising a $300 sax as a Professional Model, and not even telling you what brand it is, I'm going to say they're full of BS. If you're buying on-line, at LEAST make sure it's a reputable sax brand (Selmer, Yamaha, Keilwerth, Yanagisawa, etc.). It's always best to even be able to play a sax before you buy it to make sure it's a good sax, so ideally you don't want to purchase on-line. Your best bet would be to look around locally for a used Selmer or Yamaha beginner sax. Tenors of this type often run around $500, but if you're lucky you could find one cheaper, or if you're not set on tenor altos tend to run closer to $300. Just make absolutely sure it's the sax you want before you buy.