How big a torsion bar nick is ok?

I would post photos like you did and clearly state the damage. Don't clean up, so the buyer will know. I am guessing ~5 mil depressions. My guess is that after smoothing they would add negligible risk of breakage. When T-bars break, it is not terribly dangerous. That side just drops to the bump stops. It happened to many when poorly welded T-bar aft supports broke (1970's cars) or rusted thru. Indeed, even when LCA's break, usually nothing happens since one can still steer and that usually happens in low-speed turns. Mopar LCA's don't break, but newer cars like M-B have hollow sheet-metal LCA's which rust thru from the inside (look pristine until they fail).

On ebay, all the details in the world doesn't seem to help since many buyers are total gomers or viewing on a phone (like looking thru a peep-hole at your ad). I once listed rare Sanden compressor brackets for a 1977 Chevy straight six on ebay, w/ ample details. They buyer asked questions like "what these look like?" after buying them. The ad had 4 photos. Then nothing but bad feedback, "no fit, I'm mad". Never even described his vehicle or engine. I hope you have better luck. Unless desirable ones, they may not be worth shipping. I bought a pair of thicker ones for $10 at a swap meet, and the seller had 6 sets to choose. One can buy new ones fairly affordably.

Which T-bars are more likely to break has an interesting answer. Thicker T-bars can support more load before they break, but thin ones can deflect more before they break. Thus, either can break under certain circumstances. In Dukes of Hazzard jumps where the control arms bottom out on the bump stops, one is imposing a fixed rotation. For this, thin slant six T-bars are more likely to survive. That is the most likely failure scenario. BTW, the DofH show ruined many cars in filming by bending the frames. As an aside, what is different between T-bars and coil springs? A coil spring is actually a coiled T-bar. The metal twists as the spring compresses, as engineering professors demonstrate by coiling their belt. Coil springs have an added shear force in the weight acting thru the spring, whereas a T-bar sees a pure twisting force. Many people have gone out in the morning to find their coil spring snapped during the night. Not sure why (corrosion, earthquake?).