See this for more explanation: https://www1.avalara.com/trustfile/en/blog/commingle-not-com...
But yeah, some people do it and it's a big part of the counterfeit problem. As a seller you have to have to weigh dealing with counterfeits vs shipping logistics. "Your products take an extra week to get to me? Forget it, I'll buy from that guy."
I don't think there's a way for a buyer to know without messaging the seller.
> Used & new (28) from $19.99 & FREE shipping on orders over $25.00. Details
it's not really indicative of anything other than that someone else is selling it.
As a seller, I would much rather you contacted me before buying. My average response time is 8 minutes and most big sellers I know are <3 hours.
1. Click seller name
2. Click contact (seller)
3. Type "Is ASIN BXXXXXXX commingled?"
4. Hit send.
Looking at the seller rating is probably the best you can do aside from messaging or going somewhere else.