In the second sentence above that states "I'd like to buy [these]" The general rule is that if you use a singular noun you must use a singular verb and if you use a plural noun you must you a plural verb. The word [these] is plural distributive pronoun and the two verbs "like" and "buy" are singular. Why is that please?
Not singular, infinitive. And infinitive verbs don't change their form.
The basic rule is that the subject must agree with the verb in regards of number and person. For simple present tense verbs, that mean changing the form when the subject is third person singular (he/she/it)
In this case the subject is "I". That is first person singular.
The verb is a bit hard to see. The full verb phrase is "'d like to buy"
You can understand this to be "would like to buy" This is a chain of verbs. The first verb and the only one which agrees with the subject is "would". However this modal verb is invariable, it doesn't change form at all. This is because it has the form of a past tense verb, even though it doesn't express anything to do with past time. All past tense verbs (except was/were) are invariable.
After that there are a bare infinitive "like" and a to infinitive "to buy" which are just chained together. But infinitives are always invariable. They have no plural or singular form.
The plural pronoun in the predicate "these" is completely irrelevant to deciding the form of the verb. Here are some examples:
I would like to buy these.
He would like to buy these.
They would like to buy these.
I like to buy these.
He likes to buy these. verb changes to likes
They like to buy these.
I like to buy it.
He likes to buy it. verb change, the change from "these" to "it" is irrelevant
I liked to buy these.
He liked to buy these. no change in past tense.
I can buy these.
He can buy these. modal "can" is also invariant
They can buy these.
I buy these.
He buys these.
They buy these.