The thing about Flash memory is that it doesn't store 1s and 0s, it stores charge. When a Flash memory cell is read, the amount of charge is interpreted as a 1 or a 0. Even if you "erase" a cell, it is possible for somebody with the right equipment (e.g. law enforcement) to measure residual charge and make a statistically significant guess of what value it once held.
Flash memory cells also have a limited number of write cycles. These days they number in the hundreds of thousands, but they're still limited (the first time I designed with Flash memory, that number was more like a couple of thousand writes). In order to maximize the life of the entire chip, the on-chip drivers will use write leveling so the writes are spread across the array. Bottom line: If you write to the same location from the outside, internally it may go to different places on the array. Apple removed the secure file erase capability from macOS X a couple of years ago for this very reason, because it wasn't reliable with SSDs.
Overwriting an entire device might be more successful in making sure you hit the data you want to hit. The most secure utilities I've seen will use multiple passes (5 or 7) with pseudorandom data.
My advice to the OP is to format the cards as best as you can with the utilities you have (or can reasonably get) and don't worry about it. I would think most people around here who buy used memory cards just want to use them for their own purposes.
In his first post, the OP talks about selling some cards for "a few bucks", so it doesn't sound like this is a significant amount of money. If you're really concerned about somebody recovering what was once on the cards, don't sell them. Grind them up in a good quality paper shredder. Or smash them to bits with a hammer.