Mobile HQ
2x Arty (Prefer snipers)
[Side note to Rob: I looked back through the previous markets and didn't see arty or a mobile HQ. How often do they come up?]
For the latter, incredibly rarely. Mobile HQs and Mobile Field Bases are almost functionally extinct at current time; the VERY few which are left are almost exclusively owned by the Houses. When the Dragoons came into the Inner Sphere in 3005, their 5 MHQs were canonically (as per TRO3039) a significant boost to the number of surviving examples. They become somewhat more common post-3052, and become something you'd reasonably expect to see every 6 months or so on the market by 3060.
Currently, era modifiers keep them at the smallest percentage for AtB's marketplace generation system; any given marketplace entry has a about 1/10th of a 1% chance of being an MFB or an MHQ, if my numbers are accurate.
For the former, rarely, but not *as* rarely. Granted, until now I've had the possibilty of even finding artillery via the marketplace turned off, because I was holding to see what the BattleMech Manual was going to do about support force rules. I'd expect an artillery piece to work its way into the marketplace every 3-4 months, assuming you guys are in an area of space that isn't a shithole (read: marketplace results take your rough location into account, though not to the level where if you're near a factory, it gives you lots of stuff that that factory makes; AtB isn't *that* detailed AFAIK).