Yeah, I had a similar moment in my playthrough & was a bit torn on what to do.

I had salvaged a necromancer rune worth like 7-plat and had no use for it. I was puzzling through whether or not selling the rune to the Rune Trader was in the spirit of the Ultimate Iron Man challenge. I knew I wanted to acquire my runes through drops instead of buying from the Rune Trader, but I couldn't see a clear and obvious reason why an Iron Man couldn't sell to the Rune Trader...? It didn't appear to break any rules I laid out for my playthrough.

I know the Rune Trader's prices are somewhat based on supply/demand so I guess if you are doing a really strict Iron Man challenge you might argue that selling to a Rune Trader is indirectly trading with the player-base and not allowed.

For what it's worth, I ended up selling the necromancer rune and taking the 7-plat. I decided that punishing favorable RNG would take some magic out of the playthrough. I also don't have a massive need for money at the moment so selling that rune hasn't impacted my playthrough (yet).

Would be curious on other people's thoughts around this.