I've gotten into the habit of selling things to them because they prevent me from being a total dunce and selling things I can't buy back. I don't really mind not getting as much money back. I remember selling a silver shovel to Reese as well as one of my Brewstoids.
I explained why. I have a tendency to rush through selecting things to sell, and selling to the Nook kids prevents me from selling something important accidentally.
I already have more than enough bells to get whatever it is I need. Fish and bugs still go to Re-Tail, but when it comes to merely emptying pockets full of useless gifts from villagers, I go to the Nooks.
I sold stuff to them usually because it was just alot faster to sell stuff there. Retail was in the bottom half of my town and since I buy new things I don't have yet everyday from the nook brothers store I find it much more convenient. I don't need the extra money so I don't bother.
Unless I'm selling high value items I tend to just sell to the shop that's closer to where I am. Although your point about Timmy and Tommy not taking "important" items is a good one I hadn't considered before. I might start selling to them more now.
When I start playing I don't know what is Retail doing and always sell to Timmy Nook until I found out Retail pay higher! bye Nook! Never sell anything to you anymore!
I had no idea you can't sell 'important' things to the Nook brothers. Good to know! Though I tend to sell to Reese since Re-Tail isn't that far from my house and I don't want to run to my Main Street to sell my items there.
I tend to sell things to the Nooklings when the only reason I bought it from them was to catalog it. I had some items today I wasn't sure were orderable or not, so selling to them helped me out with that (they're grayed out and can't be sold). Be careful, though: I went back to test it, and they WILL buy Saharah wallpapers and carpets.