I have some OP Saks going on 4 years old that still hold their seal just fine. Having said that, I looked at their website. If you look in the FAQ section, they will only replace failures within 30 days of purchase, not that the bags have a life expectancy of 30 days.
If the owner actually said this, then you should request he change the FAQ section on his website as the life expectancy is listed as follows, without any specific time frame:
5. What is the life span of a bag?
There is no way to determine how often any one individual will use each bag, nor how brutal they may be with it. The better care you take of the bag, the longer the bag will last. The bags are disposable and must be replaced after extended use. Our bags are like batteries, Band-Aids, ink cartridges, light bulbs, lip balm, elastic gloves, …they have a specific purpose and a life span. When the bag is becoming worn, it is time to replace the bag.