Me gusta comprar una guitarra vieja into plural form?

The plural form of "Me gusta comprar una guitarra vieja" is "Me gustan comprar guitarras viejas."

Here's the breakdown:

* Me gusta changes to Me gustan to agree with the plural noun "guitarras".

* una changes to guitarras to become plural.

* vieja stays the same, as it describes the guitars.

So the full sentence in the plural form is: "Me gustan comprar guitarras viejas."