So basically the Estimation thing tries to estimate the value of your specific player by comparing him to the prices of players with "similar" skill sets. They do this by picking a few primary skills for your player (e.g. respectable JS) and then comparing that to 'recently sold' players who also have similar age/potential, with a similar level in that skill. However, this method has several flaws. For example, a well-rounded player will always be misjudged by the TPE, since the TPE is only comparing one or two primary skills. Secondaries will drive up the price considerably, but won't be evaluated by the TPE (imagine the difference in price between a center with 1 OD and 1 passing and one with 8OD an 8 passing). Also, it may compare the price of your player to a player sold at the beginning of the season. Your trainees right now have just about finished their 18yo season, making them essentially 19yos from a training standpoint, but the TPE is comparing them to 18yos fresh out of the draft with their whole 18yo season of training before them.
So those are two major ways a TPE can mis-evaluate a player, which is why most experienced managers consider it only a very rough guideline. In your specific case, the TPE is high because the players are 'old' 18yos but likely being compared to 'young' 18yos from the beginning of the season. Also, both players have poor secondaries (especially passing and IS on Marshall), which means that their true value is lower than some of the players they might be compared to. That's why the TPE is giving you a value that is too high for your player's current market worth. I hope this helps clear things up for you.