The qualifying list is not quite the same as the free list. There are some products that can be use to join the GB but not be chosen as a freebie, for example ARC System 3 with Mic, the Electromagnetik and the Cinekinetik bundles. None of the Max products are in the GB. The link Charlie gave you has all the information regarding the GB. You cannot sell any of the free items you receive in the group buy, but I believe you can sell the item you used to get you into the GB after 90 days.
The Custom Shop is software you can install using the IK Product manager and allows you to buy certain items for Amplitude, T-Racks and SampleTank, or even the software itself. Not everything you can buy in the custom shop qualifies you for the GB, so you have to refer back to the above link. You can buy gear credits to use in the Custom Shop. It you buy 150 or more gear credits, you can get a discount. For example 200 gear credits costs you $160. You can even use Jam Points to reduce the cost of the gear credits further. If you have enough Jam points (48), those 200 gear credits would cost you $112. You can then use those gear credit to buy something like Amplitude 5 which would get you in at the $200 level.