In the current game, there is no reason to have points in a lot of hunting skills past a certain level. And the massive excess some players have above this level would mean rock bottom prices for those skills. I'll use Messi as an example even though I feel like he would be hard pressed to let go of skills unless he wanted to switch professions or decided the level of profit he has isn't satisfying enough. The way that skills work, every 1000 points is roughly double the skill necessary to equal the previous 1000. For example, going from 4000-5000 is about 81 ped tt of skill, whereas 5000-6000 is 161 ped TT. This exponential slowing of skill progression means that assume Messi has 35000 points of shortblades. , taking out 1500 points of each skill necessary to gain profession levels in shortblades would probably be enough to get someone to level 100 shortblades. I don't know the amount of shortblades that require over level 100 and are actually the type of weapon someone would want to use, such as a prism of destruction, but I believe the number is below 10. This means that Messi alone could trade enough skills to max every high level shortblade available, and still be able to use his Unique Sacrificial Dagger. Assuming this many players are grinding high level shortblades now, this excess skill only increases further devaluing the skills, which essentially gets rid of the grinding part of Entropia. (take note that this is conjecture about the sheer amount of skills Messi has and only and educated estimate, I did not actually do the math.) If there is no reason to grind, then the majority of players will be unhappy with their losses low-mid level because they will no longer feel like their losses are justified by working towards a goal of better ROI or potentially even profit in the future. If low-mid level hunting activity decreases heavily, then the economy of the game collapses, since most markup is based around generating materials and items for this level of play. The empty skill implant is essentially a form of a check to ensure that this all cannot happen. Empty skill implants also provide hunters with valuable markup that otherwise would not exist at all. Other than that, the entire event system that is meant to keep competition fair and allow players to have a chance at more nice items and winnings throughout all level ranges would become completely useless as players with better equipment and funds would just drop to whatever rank ensures them the most winnings. Killing any chances of those without a lot of funds climbing and becoming more successful through events.
TL;DR : You said: Many ubers have more skills than they need, a lot more. There will never be a L200 gun (probably). Messi and Girts could supply skills for years by chipping out unnecessary skills. If we made a Skill Exchange, they'd flood the market.
My answer: No, they wouldn't not if the player base was to expand. THey'd also not sell too cheap, because skills cost peds to aquire.
The solution:
1. MA makes a Skill Exchange ingame. You can put your skills (in peds) up for sale. Just like now, MA takes a cut of 10% of the value.
2. To be able to purchase skills, one would need Skill Exchange Tokens (lootable), of course linearly scalable with skill size (measured in peds).
3. Minimum sale would be 3 ped of skills (this will ensure no flooding of stupidly low sale threads, and also be an incentiv to skill at least 3 ped (from 0 to 1000 skill points) to be able to sell.)
4. MU is contained in loot through Tokens, but spread out more (could be uncommon drop, like the FEN tokens).
5. ESI would exist in parallell to this system. THey would lose some MU, but still have use for transactions below 3 ped.
This way, a lot more skills would be traded, making MA more money. New players will be able to boost skills cheaper than today, and thus be able to spend more peds, making MA more money. (Remember, these are the ones to feed the top of the pyramid scheme, so ubers should welcome them)