Torn is a wonderful free market, there are very few interventions/price controls set by the devs, so the prices on things does a great job of reflecting the general pressures of supply and demand.
What does that mean for your idea? Well it just wont work. Someone can buy up all the stock right now, but unless they control the means of production they will only have a temporary impact, unless they buy up all supply for all time, which would end up being unprofitable because of my next point. So the next factor, lets assume that someone does perpetually buy all stock, and tries to sell things way overpriced. Will people buy it? Generally speaking, yes some will, but most wont. That is how the free market works, if you overprice something you will drive demand down, so you will sell fewer DVDs. Its not like every player had unlimited funds accessible to them. Players will either stop using the DVDs or will find cheaper alternatives like candies.
Now if someone was able to control the production of a given item, then yeah they can set the price because they have a monopoly, but in Torn that is impossible for all the general consumables that people want. There are so many sources of new items that it is impossible for anyone group of people to lock down the supply. The best you can hope for is to temporarily disrupt a market, sell quick, and make a short term profit. But people will immediately undercut you so you are taking a risk to try it.