If the seller is a nice person then they'd more than likely be willing to sex one for you, or at least let you bring in the exuvium once it molts so they can sex it.
As for the enclosure height, it's because the two species you chose are terrestrial species, and although they are able to climb and will do so readily, their heavy-set builds would not do well if they were to fall. The GBB you can afford a teeny bit more height because they like to web up the cage a fair bit, but even for them I'd be scared to go much higher than 2 legspans because they're built far more terrestrial than arboreal.