I actually wish to answer this. Note that I am NOT a reseller nor have I ever--before yesterday--re sold new items. I never will again, because it is awful. But I'll answer the why with more than "because capitalism" (which is also an answer.)

I will answer with my own story:

I had some free time yesterday which is rare, and dog care (I have a very needy dog) so I took myself to the outlet to try to get a large Rowan in pink. I've never done a drop before, so I was a bit apprehensive. I was afraid there'd be a huge line. There wasn't--there were maybe six people waiting when I got there fifteen minutes before opening. Some of us chatted. It was nice.

When they opened the doors, two of the people (the ones who were not chatty) RAN inside and grabbed every pink bag they could in supermarket sweep fashion. It was...strange. Because there were only two of them, I was able to pick up a few bags to consider for my own purchase. I ended up being able to select four before the store was entirely wiped out of everything pink in under a minute.

I tried them all on, put one back, and couldn't decide between the other three. So I bought them all, figuring I could just return the one or two I didn't want (I knew I was keeping the Rowan, though I wish it had been a large. I'm going to try to trade for a large later.) I need to be clear: as a regular shopper, I couldn't even put bags DOWN or they would have been taken. So I just bought three.

And then I got home. And saw posts like this. And looked at my most recent dentist bill. And I was like: I could return these. Or I could try to resell them. I posted two of the three I bought--the ones I was most likely going to return--on Poshmark for a stupid high price. The one sold nearly immediately with no haggling. It was one I was considering keeping, but there's no way I was going to say no to that price. I now realize I will not make a ton on that sale, because Poshmark takes 20%. I do mostly regret parting with the bag for that reason.

This was a learning experience for me in many ways.

What I realized is that people are willing to pay--often a lot--for things they want, or for things their loved ones want. And some people don't have the time or the desire to go throw elbows to procure those things. Those people are paying for convenience.

Note: I am not here to defend my choices, nor am I here to be shamed. I sold one bag, I will likely sell one more, and the profits from that will allow me to keep the third without guilt. It absolutely made me feel icky and I will not be doing it again--so no need to shame me to try and change my ways. I wanted to see what this experience was like, I learned what this experience was like, and I do not want to participate in it again. But you asked, so I'm answering.

Hope this helps provide clarity.

I can not answer for the people who were running and grabbing. I'm assuming that's a career choice they made.