Seller presumed to have warranted its title as good

https://b-ok.cc/book/1225779/037f5d
Under most circumstances, is a seller presumed to have warranted its title as good and valid to goods it sold to a buyer?
Is A discharged from an agreement after he co-signs B’s credit application and B then agrees to a higher rate of interest without telling A?
What would require a president’s personal guaranty to pay a loan for her business be in writing if the business defaults on the loan?