In my mind, make it with little icons that light up blue when you have them. Weapons in one row, equipment in another. Buying an item automatically removes any conflicting items. Buying an item you couldn't afford wouldn't sell your existing stuff.
Would make binds easier too, as "buy lcannon" would do the same work as "sell weapon;buy lcannon". Maybe make up specific bind shortcuts.
bckit - Best CKIT. ackit if available, otherwise ckit.
harm - Helmet and light ARMor.
jetharm - JETpack, Helmet, and light ARMor.
battharm - BATTery pack, Helmet, and light ARMor
(Capitalization used to demonstrate pseudo-acronym derivation)
While we're at it...
builderbest - Best Granger. Builderupg if availible, otherwise builder.