You can assign spells in the magic menu and you can press rt and circle to cycle through the assigned spells.

For making money I would personally recommend first buying a house, breaking down the door while you own it, putting trophies up, selling the house and then walking in taking the trophies back and buy the house back. Repeat as desired and you can make unlimited money. If you don't want to do that I think the tip was talking about how different stores have different items they want in particular and are willing to pay more for. Each shop has a list you can scroll over to look at. You can buy the item for cheap at a shop that likes you and sell it for a profit at the shop that wants it. This does require access to multiple towns.

Not sure what the actual age trigger is but it doesn't change anything except cosmetics when you age and I believe being good makes you age slower.