Basically stuff that has a restricted or limited supply. On the low end, stock up on Romulan Ale and Dosi Rotgut because the Starbase Bartender unlocking project needs 250 of each.
Right now, I'm buying up stuff that came out from Tholian Lockboxes. Warfare specialists, Subspace Jumper console packs, Phased Tetryon packs, Orb Weavers, Recluse Carriers. When the Tholian lockbox is gone, prices on these stuff should rise.