How to purchase a portfolio and avoid tax penalties

[Michael Liddicoat]

I'm a realtor who helps investors to purchase income properties.  One of my clients is looking to purchase the entire portfolio (51 doors) from a seller.  The seller is looking to avoid paying a large chunk in taxes.  

I have limited knowledge of buying a portfolio.  Are there strategies that would be mutually beneficial to both sides other than a straight purchase and sale?

Could the seller put the assets into an LLC and then allow the buyer to purchase the LLC over an X year period work? I'd like to be more knowledgeable about creative strategies and appreciate any feedback.