If you're buying a video game just as an investment, then you're taking a significant gamble with anything NGF related. Collectors set market value, and just because Neotropolis has a copy of an NGF KOF2000 does not mean it will hold its value in the eyes of any other collector.
If you want an investment, and are willing to spend a bajilion dollars to put a plastic case inside of another plastic case to be sat on your shelf and never touched again, seek out an authentic copy of Metal Slug or any of the other numerous high-priced collectors/investment items in the Neo-Geo library that not only carry more clout, but more long-term investment potential for collectors.
NGF stamps, certificates, and any other bull they tried to peddle to sell their mutilated copies of games is the riskiest "investment" you could make if you're going after the upper echelon of collectors items in the Neo-Geo market.
This is, honestly, the only serious answer you'll get on this thread. Even then, I feel like I'm being trolled...