1) ACME Trigger Group is a Coolness 4 / Sci-fi item. Do you have Sci-fi flipped on?
2) If I remember correctly, those rifles are iron sights only.
3) A lot of newer (nonSci-fi) stuff is not buy able used, though older stuff and rare prototypes should be appearing though many of the later are flagged as Sci-fi.
4) Yeah that doesn't look right, at a glance those should be under the AR tab.

EDIT: 4a) the MSBS receivers seem to be missing a <ubWeaponType>, meaning it is treated as 0. I think adding <ubWeaponType>6</ubWeaponType> should fix those.