The reason Group Buys are not allowed is that they fundamentally represent a business venture, although it is an alternative to conventional marketing.
Many Group Buys start off with basic intents, usually a talented individual with a great idea that a group of enthusiasts are willing to purchase. The person who manufactures the item makes a few dollars and the customers enjoy a unique mod/item.
Other Group Buys are essentially a means to overcome challenges with traditional distribution and marketing methods. The common example being an established manufacturer establishes a relationship with an individual (agent) who creates interest for an existing product. The agent contacts his friends, posts on eBay, and in motorcycle forums. Sales from this effort would be rewarded back to the agent.
Neither example is considered negative, but they both represent a business venture.
Which then leads to the fundamental policy, all business ventures pay to advertise on Rat.Net; which includes the ability to post about their products and/or services. If one business venture is allowed to post freely, that circumvents those that are paying to keep this site going.
I realize that this is a difficult concept for some situations. While I empathize, there is the need to keep Rat.Net funded. No one cares how much it costs to keep Rat.Net going, just as long as it is going.
Some folks would say that their mortgage is their single largest personal expense. In my case, I tell the wife that she doesn't need to worry about infidelity because Rat.Net is like having an expensive mistress - it costs a lot of money and still makes me giggle when she isn't around
Hope this helps,