There's pros & cons to this, & I think your point is a good one (many BPs need new homes! other snakes too, for that matter) but here's a few "cons"":
Many first-time snake owners are intimidated by the size of an adult ball python that they haven't "grown to know" & feel comfortable with. Not all, but some.
Many adult BPs are MORE likely to fast for a long time when re-homed, presumably because as older & experienced snakes, they're more aware & stressed by the
changes in their environment. New owners and long-fasting BPs are a very frustrating combination...though I'm assuming the buyer is once-again 'new' to snakes.
I actually recommend first time snake buyers get at least a yearling snake, one that's well-started, IF we're not talking about BPs. But BPs are a bit "different".
Another risk to buying an older snake is that while some are healthy & well-started, others are for sale because their owners "know something you don't"...like they
were exposed to something contagious or that the RI they've been treated for in the past keeps coming back...(ie. they have a resistant issue, or the start of an
RI that a new buyer may not notice right off). Just saying...there's many great snakes available that aren't hatchling/neonates, but just be careful, ask questions, &
do your research...it's always "buyer beware" anyways, for snakes of ALL ages...and well-started snakes can be a real blessing as long as they're healthy.