best way to earn points here?

You can earn at least a couple million per day by questing. There are also the games (though those are more for fun and won't earn you a lot of sP), and selling items from your shop (by questing you'll end up also getting a lot of stuff you can use to stock in your shop).

awesome, thank you so much c:

in terms of time vs sP investment, restocking from all the shops will make about half as much as quests do. But quests are limited, and you can restock all day if you like.

Vending is rarely profitable, but if something new is coming out of a shop that vends, you can turn your WT into a good amount of sP. The advantage to doing this anyway is that it will give you lots of basic stuff for cheap and there's an Achievement for it.

Is there a way to know how much things are worth for restocking?

experience!

There is a way to learn this the book-learning way. There is a fansite called SubetaLodge that does price tracking. Also the economy here has been socialized with rarity-based price-supports. This means that no matter how useless or ugly or completely unwanted by users an item is if it is a certain rarity and falls below a not-told-to-us-but-figurable price, it will vanish from shops until users are forced to pay other users a higher amount for the item. SubetaLodge will also help you sort items by rarity.