I've looked at this idea, it's possible to make money producing and selling seed. As with any business, results will be greatly dependent on the time and effort you put in, a sound plan from production through delivery, the quality of your product, the value received for the price charged, and customer service.
Most states require a license to sell seed. The cost will range from a pittance to a few hundred bucks. Some states, Florida being one, have a floating license fee depending on your sales volume. If you are selling seed based on weight, you may be required to have your scale inspected. In Florida, seed packets must include a tested germination rate. You'd need to start at least 100 seeds, count up how many showed up, print the date and results on the package. Since seeds have a shelf life, the date the seeds were harvested would need to be on the package. Debris in the package is an issue. The seeds must be clean, with tolerances stated in the state regulations for allowable amounts of debris. It would also be wise to limit your mail order sales to your own country lest you unknowingly violate some import ban and create an international incident.
Different sizes of package can work for you. Small pack would be
enough to try out. Next size up would be enough to plant a 100' row. The next size up would be enough for a market crop. It is typical that with each increase in size, the price per unit is reduced.
The highest quality of seed will demand the highest price. Anyone can sell a tomato seed. If you can produce open pollinated seed of heirloom cultivars with all natural or certified organic methods, you'll get a premium price.
Being a one man operation, I'm assuming you'll be doing the work yourself. Some seeds are easier to work with. Corn is pretty easy. Count em out, stuff a package, they'll be clean, tidy and accurate. Some are not so easy. Ever try to clean broccoli seeds? I've done it with 2 methods: winnow to get rid of the big stuff, then sift to get rid of the small stuff. Since seeds come in different sizes, you'll need different sizes of sifter screens if you are using this sort of method. Next is scales to portion the seed for packaging, unless you intend to count out 250 mustard seeds, 500 oregano seeds. I can't even see oregano seeds.
You'll need to consider crop production in regards to cross pollination from the surrounding area, not just your own field. Corn is pollinated by wind and can travel great distances. There are corporations out there with patented genes that are spread by the wind, resulting in much consternation in the seed industry. Corn is tough.
Can you pull in a few grand extra? Sure, why not.
Is it worth the time and effort? Maybe, that's your call. It will take more time and effort at the beginning, and that's where the least money is. Perseverance has it's place.
Is this a naive idea? I don't think so, people have been growing crops for 10k years, and have started with seed every year. Seed is the fundamental building block of agriculture.