As Evil ZoneE has mentioned, the Goblin Merchant does indeed sell Glue sometimes. Glue also has a small chance to drop from Slimes at from Construction Satchels.
Traveling Merchant only sells food, food recipes and the Fur Coat at the moment.
There will likely be a Slime Heroine at some point. Just probably not a Blue Slime but some other colour, though this isn't planned for a while yet.
"Headgear" was just the term I used for that slot, both hats and Cloaks go in the same slot and Cloaks are implied to have hoods that cover the head. I suppose I can rename it to something else but I would need some "term" that describes it better!