DoS 2 really needs the vendor approval system because of how fast all of your gear becomes obsolete, sending you back to the vendor for an extended trading/upgrade session.
It can’t really be called an exploit if it was intentionally coded into the game, but it still feels that way since it’s never actually explained. Thing is, I think that’s a design decision rather than oversight.
There are a bunch of systems DoS 2 never bothers explaining; rather, it expects players to mess around and find out. The upsides are a simplified experience for new players, the joy of discovery for intermediates and a more satisfying game for veterans. The downside is building an entire system and possibly letting it go unnoticed. BG3 doesn’t have that downside, as the trading system was ported as is.
I can attest to the upside. My initial ignorance was bliss, as I could focus my attention on more important matters. When I tried maxing out a vendor’s approval, I was thrilled to obtain more goodies than expected. Since then, I’ve been looting a lot less because I know how to get the most value from it.
None of this excuses the inventory issue. As was mentioned above and a million times elswhere on the forum, switching characters during a sale shouldn’t change the items’ prices.
If a charismatic bard can seduce a vendor by handing them a bunch of free daggers one by one (so that each approval bump impacts the value of the next gift), why can’t the bard point to their friends and say “they’re with me”?
