The "to" is part of the infinitive, the verb that follows it (to buy). The infinitive is considered a noun when it is used that way, or the whole infinitive phrase (to buy this) is. The "to" has various names (I call it a particle), but it is definitely not a preposition.
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Guyper"I want to buy this"It’s a subordinator (or subordinating conjunction), like that in He says that he wants to buy this or whether in I wonder whether it’s still available. These
is the to still considered a preposition then or does it render and become part of the verb that precedes it?