Group buys are where somebody here contacts a manufacturer/distributor and negotiates a low price for an item based on a large quantity order, then a bunch of us buy them. We've also done customized AR lowers with NES logos and special serial numbers.
The green blocks are rep points. If you like something someone has posted, you can click on the little sheriff's badge near the lower left corner of the post and give that person a rep point. When your points add up, you get green blocks.