Technically on the GCN one, they technically don't take your stuff, they either give you money or an item- a forced trade, it's not like they literally rob you.
As far as them taking flowers, if you have the beautiful ordinance, no flower will ever disappear, and for the ones that show up in retail, those items are just as random as the Sloppy/Cardboard, and now can confirm it, as I had Rooney say he bought some white pansies on spur of the moment, and wanted to know if I'd buy them. Plus, to really confirm, I have the beautiful, and flowers all separated by color/type. I only have like 3 blue roses when one appeared in Re-Tail. I checked, all 3 were still there, so it's truly random, they don't steal flowers and put them in their house/Re-Tail.
Lastly, if you have stuff on the plaza where Isabelle or Redd, Katrina, etc, setup, those items will either disappear, or end up in Lost and Found, or totally vanish if your L&F is full. Here's a funny thing to point out too. If anyone of the Snowman family lands on flowers, they'll take that flower- literally remove it, as 4 days later after they melt, it's gone.