If we were to buy a focusing crystal from someone would it be possible to build a Lightsaber with it if we were a force user?

Yes if you are a Force User, and have acquired the right components, and of course have the right skills, you can build your own lightsaber

Alright, two questions then. Where would we get the components, and when you say the right skills do you mean our ability to make lightsabers would take up one of our five force abilities?

No the ability to build your own lightsaber would not take up a Force skill but the number of Force abilities and the level would be a good guide. If you are just a beginner, likehood is that you can't build one yet.

As for where you can find the components, I'd say for now, there aren't any to be found quite yet, only crystals. In time we will add them up for sale, this will happen once we have more Force Users

If you weren't a Jedi, but you inherited a lightsaber, or found one/ stole one/ aquired on somehow...could you still use it as a weapon, even though it would be far less effective because you are not force sensitive.

(For example, Han Solo used Leia's lightsaber in a comic book I read once, and he's not force sensitive)

Would this be possible, or not?

No you can't inherit lightsabers, and even if you did acquire one, you can't use it as a weapon. You wouldn't have the skill to use it unless you were going through training in either Dark Jedi or Jedi.