for what is it worth - i feel too many people overpay for "potential". unfortunately, market price does reflect this.
if you have a starter potential player (like yours, smallfries), here is what I have observed, though only with a very limited sample size.
As the player gets older, if your secondaries are decent, and you can get the primary skills up, then you can sell him as a "complete" player without much of a discount compared to higher potential players. If your secondaries are too low, even compared higher potentially players with similar primaries, the "market" price will be lower.
So if you are training a starter, my suggestion is either make a complete player and sell, or keep for life.
A player like Martinez, I would personally train and keep. He will be a nice bench player once you get to div 3, at least. And he can start for you on cup games to win those. I would not train to sell, because his weak skills are rebounding and inside shot, and if I was going to sell him, I would want to dangle those as "carrots" to offset the low potential.
above is just a suggestion. Nice job training him to this stage, by the way.