I work for MercadoLibre. Now, I understand your problem more generally as "I live in country X and want to do remote business in country Y, how do I get my money over from Y to X?"
It's not as easy as you may already guess, but here are some tips, specific to Argentina and MercadoLibre:
* Have money deposited into your MercadoPago account -- it's good for you and buyers -- and use that as your "bank"
* When you need to withdraw, withdraw to someone's Argentine bank account via their CBU -- it can be any account name (does not have to match yours) and it does not have to be the same each time
* Do whatever deal you have with that person, through legal (or not-so) means:
* Legal (or at most gray) => have that person/friend/relative/girlfriend/associate transfer out of their own USD account (I assume some expats here could do that); Or, depending on your timeline as things may get easier with the new President, have that person transfer those pesos to your USD account in the US directly.
* Not-so legal => read the other posts about cuevas, but your risk and choice. I don't actually know how this could possibly take place since it would require linking someone's bank account and I'm pretty sure any cueva is going to stay away from doing this, unless we're talking about real-estate-big money. Also, the first challenge would be to get the cash to start with.
More caveats:
* Whoever helps with the transfer will have to explain to their Argentine bank (not MercadoLibre) the flow of money into their account => this will be a request from UIF (anti-money laundering govt org) and therefore may incur a tax liability or worse. In other words, they will have to cover for you. This explains why the exchange rate is higher than blue
* Related to MercadoLibre, get someone you can trust to operate for you locally. I don't know what kind of business you have in mind, but "unfair advantages" often look suspicious, and can lead to competitor vendors to report your listings, etc., to MercadoLibre. It's MercadoLibre's duty to investigate all these reports to have a fair marketplace, but what can happen is that if there is no one to call (and btw, MercadoLibre Argentina is NOT going to call you in the US), then MercadoLibre will just suspend your account.
Sounds like you are still very early in your planning. Best of luck! if this gets serious, feel free to PM directly and I can offer some suggestions to minimize fraud, etc. (there will be plenty of chances for people to take advantage of you, since you are operating in a gray area).
PS> Try not call MercadoLibre as "Argentina's eBay" -- it hurts my sensibilities, as we're so much better (really!)