My problem with Tri-Tachyon is I sometimes get a glut of midline ships that will not disappear from market fast enough unless I buy out the whole market (and then they restock several more Hammerheads and other undesirables). The point of joining Tri-Tachyon is getting the nice frigates, Medusa, and Paragon (and phase ships and underperforming cruisers), but many of the ships I see are something I can find for sale anywhere else. Also access to more heavy blasters and some other weapons (but Tri-Tachyon does not sell very many HVDs, the best medium kinetic). Currently, Hegemony is the best for weapon shopping (due to so many military markets).
P.S. I also get frustrated that Tri-Tachyon sometimes simply refuses to sell Hyperion, Scarab, or Tempest for nearly the entire game. Makes me want to attack their fleets (with transponder off) and save-scum until they cough one up for boarding.