My strategy for purchasing seeds is relatively simple. I differentiate the seeds I need in bulk from the seeds that I only need in small amounts. I then create a plan. I typically purchase bulk seeds from local seed and feed stores such as Elrod, Barnes Store, Marietta Seed and Feed, and Turner's Feed and Seed. For the smaller orders, I usually go to Home Depot.
What seeds do I buy in bulk? Most often the seeds I need in bulk are the ones which I direct seed such as radish, turnips, and mustards. These seeds are planted in tight rows, which makes other forms of seeding impractical. Also, I use an EarthWay Seeder, which performs best if a large quantity of seeds are placed in the hopper. Those seeds are cheaper at local feed and seed stores because the employees scoop the seeds directly from large seed sacks into brown paper sacks upon request from the customer. The customer can purchase pounds of those seeds if needed. This procedure eliminates the cost of fancy seed packaging.
Conversely, I always transplant certain plants such as tomatoes and peppers. Therefore, I need less of those seeds. Small packets of the seeds produce amazing amounts of plants if the seeds are distributed carefully in the seed trays when starting the seeds. Also, many of the small order seeds are of the indeterminate variety and produce a harvest for much of the growing season.
Do not forget context when mapping out a strategy for buying seeds. In Georgia, or at least the west central portion of the state, seeds are very cheap and accessible. I have yet to find it useful to browse a catalog to purchase seeds at inflated prices. Furthermore, climate and geography dictates planting methods. My area of Georgia has hot, humid summers and clay soils. I do not transplant all of my plants as recommended by other market gardeners due to the weather. I also can not direct seed certain plants due to the clay soil.