Why is 保证 in 价格还可以，就是不知道质量有没有保证?
价格还可以，就是不知道质量有没有保证。


I think 质量有没有保证 in the context checks on two aspects: 


quality. (whether the quality is good or not)
guarantee. (E.g. one-year, two-year, etc.)


It would probably be clearer to write: 价格还可以，就是不知道质量怎么样，有没有质保。