With 3.1 arriving withing the next month or two, it is time to start stocking up on necessities. If you are a raider, this means picking up consumables while they are still cheap. If you are a gold making machine, you want to take advantage of your professions and farming.
You can still sell items now as people will need them. However, keep in mind some of the big changes coming with 3.1, most notably dual spec. There are a lot of people out there that have been picking up offsets in raid content. That said, there are many who havent had a need to use the set and have yet to enchant or gem them.
You can make some very nice money by stocking up on gems and enchanting materials. Find what sells good, and figure that it will be selling even better once the patch comes out. Everyone is going to need dusts, essences, and shards for their shiny new gear in their shiny dual spec.
For gems, think ahead to what people are going to be wanting. You will want to be cutting Scarlet Rubies into strength and spell power cuts. These already go extremly well, so they should potentially shoot up in price. For yellow gems, I think that defense and hit gems will be going up. New tanks will need to hit that defense cap and ALL DPS should be trying to the specials hit cap. The beauty of hit gems is that everyone needs them. We should probably see a decent jump up in pure stamina gems due to a new wave of tanks. With dual specs will also come a need for a new set of Glyphs. Find out what Glyphs are considered "mandatory" for major PvE build and stock up. They will be selling like hot cakes in that first week.
We also have to keep in mind that the next patch is bringing us what most people have been eagerly awaiting......Ulduar. A new raid means that people will actually be required to bring consumables. Cooks and Alchemists rejoice! I fully expect the price of food to sky rocket as well as the cost of flasks. Flasks will likely be in higher demand than elixirs as people are going to want to have their buff for two hours due to wipes on new content. I could easily see these items going up by 50-100 percent for at least the first few weeks of the patch. I plan on fishing up a ton of food and storing it for the rainy day that is 3.1.
Store up items these items, and there is a good chance you could be swimming in gold within a week of the patch hitting.