I don't think of my garden journal as a journal (though it is sometimes), but more as a way to record what works or didn't, or what I think I should do differently next time. I also use it to record how many eggs I get. Or used to get. The girls are on hiatus right now.
Selling seedlings... never thought about doing that. Spring craft fairs or farmers markets might be a possibility. If I were looking at seedlings to buy, this is what I would look for:
Healthy! Above all, healthy!
Be sure they are clearly marked as to the breed they are. When I have to buy tomato plants, I want to know EXACTLY what kind of tomato plant they are, not just cherry tomato, but Cherry Sweetie, for example. I'm big into heirlooms too, so if you grow those, mark them as such.
I think this could be a great idea! Think of what you can use for pots. Plastic margarine containers with a hole drilled in for drainage? Plastic peanut butter jars, with the top part cut off? Yogurt or cottage cheese containers? If you have to buy all the pots, you will be spending too much money to make it worth doing, or make the seedling too expensive for people to want to buy.
For tomatoes and peppers, you will want to start them inside, under lights, about 6-8 weeks before they will be big enough to sell.
Good luck if you do this!