Question about tickets – newbie!

Hey all, sorry if this is an obvious question but I’ve never been to an Astros game before and was wondering:

I can see that the later season games are not on sale on the Astros site (says they go on sale in late June, v the Rays) but I notice that you can buy them all now on sites like Stubhub and similar sites.

Are these sort of sites legit? If so, how are they on sale before they’re on sale on the MLB site? Is this a good way to buy tickets?

Thanks in advance!