I noticed items such as highland tomatoes and blood vodka for sale. What do items like this do?

That items are collectable.
They serve only to collect, do not change anything in-game.

So Hector Relax isn't a real relax? It's just a very expensive collectible?

Yes.

It's just collectable as well.
All have a story behind each collectable. More I do not know much about them.


Edit: omg 2 ninjas...