Short answer. No. Article 1491 of the Civil Code bars the listed persons, including a guardian and an agent, from acquiring the property by purchase either in person or through the mediation of another. Using a dummy or nominee to hold the property for them does not escape the prohibition.
What the law says
cannot acquire by purchase, even at a public or judicial auction, either in person or through the mediation of another
Civil Code, Article 1491 — Persons Who Cannot Buy. Read the full provision →
The law anticipates the dummy
The classic attempt to sidestep this prohibition is to have someone else appear as the buyer while the barred person is the true acquirer. The article shuts this down in its own text. The listed persons cannot acquire by purchase, even at a public or judicial auction, either in person or through the mediation of another. The phrase through the mediation of another is aimed squarely at the nominee, the stand-in, the dummy who takes title on paper for the real party behind him. The ban follows who is genuinely acquiring the property, not merely whose name is written on the deed.
Who cannot buy, and over what
The prohibition applies to persons whose duties clash with buying the property in question. A guardian cannot acquire the property under his guardianship. An agent cannot acquire property whose administration or sale was entrusted to him, unless the principal has given consent. Executors and administrators, and certain public officers, judges, and lawyers, are likewise barred as to the estates, public assets, or property in litigation connected to their roles. In each case the person is dealing with property he is supposed to protect or handle for someone else, which is exactly why the law will not let him quietly acquire it through a proxy.
Why a proxy changes nothing
The conflict of interest the rule guards against does not disappear because a friend or relative signs the papers. If the guardian or agent supplies the money, controls the transaction, and is meant to end up with the property, the mischief is identical to a direct purchase: the fiduciary has helped himself to what he was trusted to safeguard. Recognizing a dummy arrangement as valid would make the prohibition meaningless, since anyone caught by it could simply route the deal through a stand-in. The law therefore looks past the arrangement's form to the reality of who benefits.
The risk of trying it
A purchase engineered through a dummy to defeat this prohibition is on very weak ground and invites challenge from the ward, principal, estate, or others whose interests the rule protects. It also damages the fiduciary's standing, suggesting he set out deliberately to evade a duty the law imposes on him. Where the law does allow acquisition, as with an agent who has the principal's genuine consent, the right course is to obtain and document that consent openly rather than to disguise the buyer. Concealment is what turns a possibly permissible dealing into a clearly improper one.