I purchased a very large block of ISBN's and bar codes directly from Bowker in 2007. At that time, it was recommended not to purchase ISBN's from third-parties. Incidentally, in some publishing forums posts appeared from individuals who claimed someone else used their ISBN before they did... that means the ISBN they had was either gotten through some automatic hack-bot from their computer, or the ISBN they purchased was fraudulent.
Bowker has changed their marketing since 2007. Now it seems that companies like the one you mentioned are able to purchase large blocks of ISBN's and resell them. That was not common in the past.
The pro to using a third party is that the cost will be significantly less than going directly through Bowker.
The con to using a third party is that the buyer does not really know where in the world that company (or malicious individual) is located. Not knowing the location is a problem if the product purchased is fraudulent. If a company based in the US, Canada, the UK, Australia, or New Zealand is found to commit fraud, it will be much easier for the Western victim to recover losses. However, if that company is located within a third-world country that is has a high reputation for fraudulent internet transactions, then the buyer will ultimately lose all costs.
The company you provided does not have a Terms of Service agreement, which should at the very least identify their primary city/state/country of operation, and their About Us page leads to a 404-Not-Found error message.
It could be that they are new and are setting up their website, or it could be a reason to be concerned.
I have been professionally formatting eBooks since 2007 (and, incidentally, have a crappy, undeveloped website -- but my clients meet me in person and find me through personal referrals). I am asked on a regular basis if I sell ISBN's, and explain that I will not because of potential fraud.
Professionally, I feel that there is a risk of fraud if an ISBN is purchased in a way other than directly through Bowker. Right now, the consumer is simply not protected well enough for my endorsement.
Personally, anytime I make an internet purchase from a new company, I do my best to research it. I follow the links in their website footers, I look through the Terms of Service to find their location, and I do general search inquiries to see if I can discover forum posts where people complain about their services.
If you are strongly thinking about doing business with that company, then may I suggest to trust your intuition. Something inside you inspired you to make the post here to ask others opinions, and to me that suggests some element of doubt is present within you. I may be wrong on that, but for me, when I ask others about something, it's because I hold a degree of uncertainty about the topic.
I hope I have provided rational and reasonable food for thought.
Good luck making your decision.