What to use to cut things on non stick surfaces?

I'd pre-cut them myself - I tend to cut mine in the tin using a plastic spatula; after all they're soft and easy to cut. Then

There are stiff plastic knives (for some reason sold for use on lettuce). They're much better than metal and will easily cut brownies.

If you did want something disposable, some of the wooden disposable cutlery is surprisingly robust, and should be safe on non stick. As you're reckoning on getting the tin back, I'd hope you'd get the knife back too. There are also semi-disposable plastic pie slices/servers (eBay link) but I haven't tried them myself.

You can't just use a ceramic knife. It's harder than metal and just as sharp, and the problem is both, but especially the hardness. Even a table knife or the edge of a metal spoon can damage a non stick coating.

You can use soft cutters for some baked goods, but not for all. Wherever you need a sharp cutting implement, the solution is to change not the knife, but the cutting surface. I have three typical options there:

• Bake with a paper layer under the batter. Probably the most useful for something like brownies and other things which stay in the tin and tend to stick.
• Bake in a tin made from a different material. Glass and ceramics are reasonably "nonstick" for many kinds of cakelike things, and I cut with knives on them. You do get scratches on glass, but they are cosmetic.
• for "official" things like a birthday cake or a beautiful torte, I take them out of the tin and serve them on a separate dish that can take the cutting. These tend to be the ones which are the least likely to hold up against cutting with a plastic knife, because they tend to be more layered.

If you want to keep the convenience of serving something baked in a nonstick tin, you have to live with unclean cuts caused by plastic knives/dough scrappers/spatulas/whatever you have lying around. Anything that is sharp enough for a nice clean cut is sharp enough to damage teflon.

Although you say that disposable plastic knives don't work well, they're frequently recommended for cutting brownies because of their naturally non-stick properties. I've used one frequently for this purpose, but I do recommend using heavy-duty ones.

https://lifehacker.com/use-a-plastic-knife-to-cut-brownies-cleanly-1760035173

That said, I wouldn't bring any container you care much about to a potluck. Remove the items from the pan, cut them into serving size yourself, and place them in a less valuable and less delicate container for serving.

Wooden knives work well, even the block-shaped ones for children.

Avoid using any other materiel than wood and silicone. However, repeated cutting or excessive pressure will wear out the surface. it's much more sensible to transfer your food to a cutting board and return it to the pan afterwards.

Non-stick care tips: https://www.thespruceeats.com/ways-to-ruin-a-nonstick-pan-1907507

Many types of Wooden knives exist: https://www.etsy.com/ca/market/wooden_knives

Adding to Chris H's answer, precut the brownies for the potluck, but...

...line the pan with foil or parchment paper, and lift the brownies out of the pan before cutting them. Cut the piece of foil several inches longer than the pan to give yourself good handles. If you do this the right way, you won't even need to wash the pan.

Here's what it looks like:

If you want to use the pan to transport or store the brownies, just place them back in the pan after cutting.

You could use a wooden or plastic spatula.

Use the end in a straight downward action, as if it's a chisel. It depends on how hard your brownies are, mind you.