Depending on what you plan to grow you don't need to buy seeds.

Tomatoes, you can squeeze the seeds out of a tomato and they will grow (better if it is a variety that is not an hybrid).
Garlic, each clove is a future plant.
Potatoes, cut them in half or into quarters if they big, let the cut part dry overnight and plant them.

What else? Unless you're starting on a very empoverished soil, as others have said I wouldn't worry about fertilisers. If I needed it I would buy a 25Kg bag of NPK and that would last me for years in a small garden. Depending on where you live that could be a bit challenging to buy, I live in a rural area so that is easy enough to find.