Daniel is not intended to be a complete source of goods. He's there just to give you a little boost. Yes, most of the items he sells are kind of pointless because you can just make them yourself, but it's nice to get the higher level items like cheesecakes or couches that are kind of annoying to make on your own. The items he sells are random, so some days you'll get great things (dozer parts, locks, Tokyo items, etc) and sometimes you'll get stuff like screws. It's a roll of the dice.
He refills his store 24 hours after you purchase his stuff. I'm not sure how long it takes to refill the items if you don't buy them. Personally, I buy them all because I'll need them eventually and he always sells at less than maximum price.
It's important to remember that he's not meant to be a major game mechanic -- if he sold 8 locks at a time as you suggest, you could just get everything you need from him and there'd be very little game left to play. 🙂
