My ruling on Profession is that it can stand in for a relevant skill at the player's option. So, if the player has Profession (herbalist), the player may be able to Appraise some plant-related items, identify some plants and even plant-based poisons, etc. This is just a house rule though that helps to make putting points into Profession desirable.
The rules say nothing about this. You're in DM territory. Personally I would allow it, and I would even venture to say more DMs than not would allow it, but I can't speak for everybody.
It depends how specific the profession type is and what sort of item you're selling. If you have Profession (jeweller) and are trying to sell a diamond ring then it absolutely makes sense to use Profession in place of another skill.
Profession (merchant) or (salesman) might even let you generalise the sort of thing you're trying to sell.