i'm currently trying to buy on seller finance. 

If there aren't any massive physical problems with the property, it appears you can grow income and equity.You might be able to buy this with little cash out of pocket if the seller owns it free and clear and was willing to finance nearly all of it.  

In the event that a seller was to owner finance a property to a buyer without a SSN, is there a market for the Note to be sold to a note buyer?  

A sellers market in general.

From their point of view they're seeking three primary things:Access to capitalControl of their dealCost of capitalThey likely have a lot of the first two so the only motivation they'd have to accept your money into their deal is either the third bullet. 

Seller won’t budge on price and it will be tight for cash flow.

We told the seller we would take the property as-is.

The seller has considered seller financing with 20% since this is a paid off property.

@Terrence StarkTypically the contract to buy the new house would specify that the builder/seller would deliver the type of plans you are referencing.

Do not get excited about getting a low offer signed by the seller as the lenders valuation will be the ultimate determining factor of approvable sale price.