Flubman
Terrarian
It makes sense thematically (The Cyborg already sells the Portal Gun Station). It's a very niche item that can get in the way of the Traveling Merchant selling you what you want. Also, it's weird how you can buy this before you can get a Portal Gun. I don't think that making the Traveling Merchant only sell Anti-Portal Blocks post-ML would be a good solution, though. Any players who want it would probably have to wait around a lot. Making it so that the Cyborg sells it if you have a Portal Gun in your inventory wouldn't have this problem.