About the Golden Shovel

Do I need to buy the fertilizers and then use them, or do I just need to buy them and am able to sell them without the fear that I screwed up since I didn't use them? All I know is that I have to buy 50 fertilizers (that's 50 days: nearly 2 months!) but none of the wikis state that I have to use them nor don't have to use them.

You just have to buy them. It's up to you if you want to use them.

Oh, good. For the past few days, I have been selling them right after buying them since they cost a lot.