You could not die, which would be pretty valuable.

But come to think of it, you could also make tons of money. An ordinary educated person today knows way more about medicine than medieval doctors did. So your optimal career goal should probably be to become a court physician. You could become as rich as minor nobility doing this, and (most importantly) without having to take sides in power struggles.

SaS = Soaps and Sterilization. Hmmm, interesting, but seems like you'd have to violate your second rule ("no opinions"). Maybe if you could manage to out live most other folks into your late thirties and forties (steering clear of all sorts of bugs - water, food, and transmission-based) the marketing would be your very aliveness. People would have to solicit your counsel rather than you having to actively sell it.

One of the commentors at Kotte's mentions that you could sell boiled water as "de-sprited water", good for all menner of medical procedures.

Unfortunately, another commentor recognized that firewood would be difficult to access, and cow poo doesn't fire hot enough.

Why would firewood be difficult to find? I could be wrong, but I don't think the European deforestation had set in yet, at that time. Is the wood in most European forests not of sufficient quality?

It belonged to the nobles.

Depends where you were. In feudalized, agricultural bits of Europe, everything did, at least nominally. But especially in 800 there would have been large expanses of uncultivated and mostly depopulated forest.

Didn't the ancient Chinese mine coal? Not a bad map to bring with you: surface coal deposits. Still, start global warming 500 years early?

There might be reasons for doing it other than making money. For instance, (1) making life more pleasant for yourself and (2) saving a whole lot of lives.

Perish the thought that anyone might think either of those more important than making money, of course...

/joke/

Still, there's an argument to be made that people can be better convinced of a need if there's something they think they have to buy.