Simply place the crop into the Seed Maker to receive the seed from that crop. Then put them in your shipping box or sell them in town. This makes you more money because, for instance, while a single radish will sell for 60 gold, a radish seed fetches 100 gold. Unfortunately, this won’t work for all crops, so see which ones it will and won’t work for.
An example of crops that sell for more as seeds than crops are Radishes, Cucumbers, and Potatoes. Amongst the crops that sell for less as seeds than the actual crop are Strawberries and Cabbage.
First, purchase strawberry seeds from Jean (about two backs should be enough). Also, buy Greenifier from Edward. Plant the seeds in Carmite Cave in an easily accessible area, and use the Greenifier to level up the crops. When they ripen, you’ll have two level two strawberries.
Sell one and make seeds with the other. Use your profits from the ones you sold to buy more Greenifier. Within one season, you should have more than 20,000 gold from this alone if you regularly continue the process. Within a year, you should have enough money to get an upgraded house from Neumann.
If you want to supplement your earnings, take your fishing rod into the second pool of water on the floor with the Chirori. This pool has Needlefish and Jamasquid, and they are both catchable without a rod upgrade and quite rare and worth some money.