what do you mean you can design the contract slightly? with disney or resale you pick a uy, number of points , and set an interest rate if financing, what else can be negotiated? what do you mean as is for the nondisney contract. please give details.
With Disney, you pick the number of points you want, they assign you a UY.
With a resale, you are stuck with what you buy. No choice on the UY or number of points, except for what is available. You can't reduce the numbe of points or add a few if you need a few more. For example, SSR contract for sale on resale, Sept UY, 143 points. If you want 150 points, you have to find a contract for sale with 150 points. With Disney, if you want 150 points, you buy 150 points. If you want Oct UY, you have to find one with an Oct UY.
With Disney, if you want to finance the purchase, they will offer you financing at either you take it or you don't. If you don't you pay for it anyway you can. With resale, you can't finance through the resale company (usually), and have to find your own financing.
When you buy from Disney, you pay their asking price. No negotiation on the price with them. If you buy resale, you can always negotiate the price with the seller. You pay their price, but they pay all closing. They lower the price and you pay all closing. You pay their price, but they pay all the annual dues for the points already used in the contract or for the rest of the year.
You might find the perfect contract with the number of points and UY that you want, but their might be no points available until next year (or the next, as in borrowed for the next year already). This would be a stripped contract. And you buy it as it. No points available for you for two years.