I'd recommend buying the hunting wagon as soon as you're able. That makes keeping Cripps stocked so much easier, plus if you're in need of cash, filling the wagon and selling to a butcher will bring decent cash pretty quickly.
As others mentioned, the large wagon is what makes the most money, but it sure takes a lot longer to fill, as well. You can deliver any size load (25, 50, 75 & 100), but only a full load pays well. The only time I'd do a small load is if a daily mission requires selling items and you don't have the time to wait for a full load.
One other tip is to donate everything to Cripps: feathers, fangs, antlers, etc. Every item donated brings in 5 XP, and while it doesn't seem like much, it does add up. Of course, once you max out your rank, there's no point in donating those and it's better to sell them to a butcher.
Sorry if that was more info than you were looking for. Thought it might help you and anyone else who comes across the post.