I think hard alu plates are harder to sell now, because more people know about the compound formula. But food, especially syrup sell well, because syrup is ideal for mass production (makes 25 everytime), so you can make a lot quick, and sell them for a cheaper price. Apple syrup, grape syrup and mango syrup some of the best, easy to make.

One way to earn money:

gather oak, or buy oak from someone, including NPC
then turn it into 2 oak wood
make some vertical wings, and sell to NPC, they give 21G each!!!

NOTE: I have did this a few times, but I made propeller instead, and it works.