This really will not help if the webmaster removes the links after 90 days. To be honest, there is absolutely no guarantee with the purchase of links if they will be maintained. Almost entirely you go for confidence. Even if you have a contract, they will be very difficult to enforce, there will be problems with jurisdictions if you try and, often, the webmaster who sells the link may not have the right to sell it, so the contract is worthless.
In my opinion, these aspects are based on trust and experience. Once you know a website for a period of time and see that your links are maintained, you can continue doing business with them.
This is the reason why I would recommend people to go to third party resellers to build their links and make the reach of the guest. That way, the reseller is responsible for any loss of links, and you can always claim your money back to an intermediary (who will personally guarantee the link from your own pocket) instead of relying on the webmaster who probably has many other strategies of monetization / business in progress.