The escribano has the responsibiliy to "certify" the source of the funds. Some escribanos are very diligent about this and some appear to be less so.
You could start "shopping" for an escribano while you are looking for an apartment. Each escribano will tell you how you must demonstrate the source of the funds to their satisfacton (for them to agree to conduct the escritura).
What they tell you may depend on the source of the funds you used to buy the metals, where you bought them, and where you will sell them. If you bought the metals in Argentina with undeclared income that was earned in Argentina, you can expect a different answer than if you bought the metals in the USA and will sell them in the USA.
If you sell them in the USA you will obviously have to get the funds to Argentina. Western Union transfers will cost about 5%(?) and you will only recieve the funds in pesos which you would then have to use to buy (suddenly more expensive) dollars. Perhaps those costs will be less than the taxes someone with tax residency would have to pay on declared income. Some ecribanos might already know the answer. If not, they probably work with an accountant who does.
If the metals are in Argentina and you sell them here, at least you won't have any costs getting them into Argentina. There may or may not be a tax on the gains/profits. If the sale of the metals is en negro and the escribano is willing to record the sale, perhaps you will never have anything to worry about...or lose sleep over.