I love answering questions on the Gardenweb gardening forums, so thought I'd preserve some of them here. This one's an answer to a question on the Professional Gardener's forum from someone who wanted to sell seedlings.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I'm doing this very thing to make a little money for a project I want to do at the library (a veggie garden to teach people to grow their own food), though I'll be having a one-day plant sale in their parking lot instead of placing classifieds. You may want to consider that, if your neighborhood allows that (you may want to approach any "powers that be" with the idea that it's like a garage sale, but plants instead of "junque"). Or maybe you can look into getting a booth at a local farmers' market? Go ask first to see if it's even worth selling plants there. If they tell you no one sells plants, that might mean an untapped market you can exploit, or it could be there's a good reason no one's doing it. Try to find out which beforehand.Milkweed (Asclepias) is a good one from your list ~ very popular and fairly easy from seed. Do you have experience with them yet? If so, maybe that should be your one plant to start with.
I know you said you don't want to do annuals, but if you're good at starting petunias, pansies and violas from seed, you may want to try just a few of those anyway. Even though everyone and their dog sells those, people who come to buy your other plants may pick up some of those as well as it seems genetically programmed into people to grab a couple six packs of those three every time they see them. LOL! Just be sure that your plants are healthy and priced within reason or you won't sell a one.
Basil ~ super easy from seed, but again, it seems like everyone is programmed to buy that when they see it. There are so many different types, and those do sell, but mostly it's the regular old type that sells. And if you don't sell them, you can pot them up into larger pots and have another chance at selling them later in the year, just like the milkweed.
Chives ~ another one easy from seed (but only fresh seed ~ germination rates go WAY down if they're old). And another one that's popular. And they're perennial. You probably already know this, but just be sure to have a BUNCH of plants in the same pot ~ little pots with just two or three chive plants don't sell as people won't get their money's worth.
Tomatoes, peppers, eggplants and tomatillos, are also easy from seed, exceedingly popular (well, the tomatoes and peppers are ~ not so much on the other two), and good candidates for potting up larger if you don't sell them at seedling stage. Tomatoes and peppers are another example of "see them = have to buy them" plants, especially the good old types people are familiar with like Patio, Beefsteak, Big Boy, Sweet Million, California Wonder pepper, Banana pepper, jalapeno, etc. And the ornamentals go well if they're already large enough to be flowering/fruiting ~ the ones with brightly colored fruit, purple foliage or variegated in any way. Eggplants and tomatillos not so much, so I wouldn't start very many of those (but if you don't, atleast down here, you'll be sure to get asked if you have any ;). If you give out printed recipes with those two, you're more apt to sell them (you can print multiples to a page and cut out to make it cheaper), but again, not nearly as many of those sell as do tomatoes, so only start a handful.
I'm also starting a few artichokes for my plant sale. Easy-peasey from seed. I'll be printing out detailed care info to give with them and will make sure people know they're a biennial, so won't likely get any harvest 'til next year (don't want anyone surprised later, thus mad at me ;). But I think they'll be a nice novelty that people may try one or two of. They do sell quite a few of them at the nursery where I work ~ I was surprised to see that, so thought I'd try them as well. Worst case scenario ~ I take them home, pot them into larger pots and grow them through the winter, selling them next year for more money since buyers will be getting fruit from them that year.
Have you looked into rooting cuttings from plants you have already? Some easy things are rosemary, honeysuckle, any type of willow, any type of fig (fruiting or ornamental) and most types of the hibiscus family (hardy ones). It wouldn't cost much to try out other things you have ~ just put the soil in a butter tub or something and stick a bunch of cuttings in together. If they don't make it, not much time or effort is lost. If they do make it, they're easily separated, even if WELL rooted, by dunking the rootball in water (you can strain out the rooting medium and reuse it even). One more tip ~ if you don't have any of these plants, ask around in your neighborhood ~ someone may give you cuttings in return for helping them do a little pruning or deadheading in their garden. Or maybe they'll trade you cuttings for some tomato plants later.
(You may have already thought of this, and if so, I apologize.) Have you added up all your costs to make sure you won't actually lose money? I haven't since I know I'll personally lose money (all proceeds are going to the library project), but am doing it more to find local gardeners and get them interested in my project, which I think an advertised plant sale will do, than for a return on my investment (though I do think I'll make a little money, if I don't count my time). If I were looking to make money for myself though, I'd definitely and carefully add everything up beforehand, not forgetting to add in the water bill. Not only will this help you avoid getting deeper into a hole monetarily, but it will help you find places you can cut back to save more. Like using recycled pots ~ I'm using ones I get from my job and a local golf course to cut costs ~ that may be a good thing for you to do as well.
Whatever you do, start out small. I can't stress that enough because if you start small, you'll learn from your mistakes instead of being drowned by them. Good luck! :)