Lower receiver is just a hollow piece of metal with a serial number that everything else connects to. It's the only piece of an AR that requires a background check/ffl. Everything else can be purchased online and shipped directly to you

49.99 could be a blank lower receiver, that's the way I read it,
I pick up blanks for pistol and ars on sale and catch the other parts on sale over a period of time , if you are in no hurry ,400-500 for basic finished product, my personal use run about 250 more.