The short answer is "yes". Only in rare situations would a seller not be able to qualify for another loan. This, of course, assumes seller would typically qualify if they were not going to leave their loan behind. Let's explore possible explanations that can be used with seller.

Straight Rental

If seller doesn't sell house and plans to move anyway, seller will be forced to either lose property to foreclosure or lease property out soon.

Yes, there are other solutions, but this is what typical motivated seller sees as their options by time they jump on phone and start contacting real estate investors. The above responses seem to be two most common answers to "What will you do if it doesn't sell?" question.

So, let's assume for discussion purposes that we are not involved at this point. If seller finds someone to lease their property, seller's loan will still be in place. The seller may or may not have landlording experience and may or may not have a decent tenant. Those arguments come in handy for other objections, but don't really affect "new loan" scenario.

Most lenders will give seller a 75% income credit toward their debt ratios. For an example, assume seller has an underlying payment of $750 and a tenant who's paying $1,000. The lender will include 75% of rental amount, or $750, as income which will help offset underlying debt payment of $750. It's not actually a "wash", but it's pretty darn close.

Even if rent were only $750, 75% rental income credit would equate to $562.50, against monthly payment of $750. In my experience $187.50 is usually not enough to disqualify seller for loan.

So, to summarize, regardless of whether you plan on acquiring property through a lease option, Sub2, or some other form of creative financing where existing loan stays in place, worst case scenario should be that new lender treats property as if it's a rental.

Lease Option

If you've entered into a lease option agreement with seller, this may work favorably for seller in qualifying for a new loan. Again, worst case should be that property is treated as a straight rental. Best case would be that lender gives seller full credit for debt payment.

Sometimes lenders have different requirements to "prove" payments are actually being made by investor. In past I've been asked to supply a letter confirming my agreement to be responsible for payment. Sometimes having seller show lease option agreement may be enough. Other times I've had to actually round up copies (front and back) of cancelled checks and mail those off.