Learn Polish: Ludzie kupujący tu nasiona to rolnicy. - The people buying seeds here are farmers.

To form an active adjectival participle, you start with the 3rd person plural present tense of the verb. For kupować, "they buy" is kupują. Then, you simply add -cy to get the masculine form (kupujący), -ca for the feminine (kupująca), or -ce for the neuter/non-masculine plural (kupujące).

Why does the plural participle kupujący look exactly like the singular masculine form?

This is a unique quirk of active adjectival participles in Polish! The nominative singular masculine form is kupujący (a buying man). For the nominative plural masculine personal, which is used for groups including at least one man like ludzie, the ending is also -ący. Therefore, depending on the noun it describes, kupujący can mean either "buying" as a singular male or "buying" as a group of people.

Why is the word to used here to mean "are" instead of the verb są?

When equating two nouns in Polish (People = farmers), you have two choices. You can use the verb są (are), which requires the second noun to take the Instrumental case (są rolnikami). Or, as in this sentence, you can use the word to, which acts as a bridge and lets you keep both nouns in the basic Nominative case (ludzie to rolnicy). Both structures are very common and natural.