"Opportunity of purchasing" vs "Opportunity to purchase"

Both are grammatical.

"opportunity to do something" will put more emphasis on "opportunity" while "opportunity of doing something" will relatively put more on "doing something", I mean in written English.

The second one feels much more natural to me. I'm not sure that there's anything wrong with the first one though.

The more normal English expression would be

chance to buy (general-purpose, conversation)

or possibly, dependent on context

offer for sale (adverts) or option to purchase (legal small print)

There's nothing wrong with opportunity or purchase, but they are a bit 'wordy'

Actually, depending on the context of the sentence, the phrase "give a purchasing opportunity" may be more idiomatic.

According to Oxford Learner's Dictionaries, both your phrases are right:

• opportunity (to do something) You'll have the opportunity to ask any questions at the end.
• This is the perfect opportunity to make a new start.
• opportunity (for something/for doing something) There was no opportunity for further discussion.
• There’ll be plenty of opportunity for relaxing once the work is done.
• opportunity (of doing something) At least give him the opportunity of explaining what happened.

So, looking at your examples:

• opportunity (to do something) to give the opportunity to purchase
• opportunity (of doing something) to give the opportunity of purchasing

It would also be right:

• opportunity (for something/for doing something) *to give the opportunity **for purchasing*