I am working on a rehab deal right now.

My attorney says he needs a bulk sale affidavit from the seller, but the seller says it is not necessary.

Has anyone been through a similar process or know if/why this is required?

Normally required from an LLC that is selling assets, unless it is their normal course of business. If it does not apply to them, they should sign an affidavit that they are exempt.

If your attorney wants it... Do it. He is trying to protect you and I’ve heard horror stories of not getting it done. 

Besides.. it takes about 0 effort on apart of the seller. Your lawyer should be handling everything and you should be taking on the expense.