Don't think of likes as "getting someone to like my page will make them more likely to buy." You need to think of them as "this increases the network of people

Don't think of likes as "getting someone to like my page will make them more likely to buy." You need to think of them as "this increases the network of people who will interact with my content, ads or otherwise, thus spreading them to their friends organically (so I don't pay)." This actually should discourage people from buying scammy likes, because that network will be substantially more valuable if it is more representative of who you want to reach.