I'd rather not inform IRS ... if they start looking at GB's ... 
Well normally (in *theory*), individuals don't make any (or at least not a whole lot of) money from group buys, they just act as the middle man and pay the actual supplier of caps. This is different because until he places an order, this is considered "income" as far as the IRS is concerned (stolen income, but that's a different thing). I am not an accountant, but this is my understanding - please correct me.
Pretty much any payments you receive are considered income, however, you can usually write those off against the expenses. So if Ivan received a total of $30,000 from the GB participants and paid GMK $29,500, he would've only showed a profit of $500 and therefore would've only had to report that amount on his taxes. Now, since I don't think a lot of people in the keyboard community consider this their job and have no profit motive, it would just be considered hobby income and you would just report what you received netted against your material cost and that also avoids having to pay self-employment tax on that income.
In this case, if he took the money but didn't pay the vendor, Ivan should be reporting all of that money on this tax return as income, albeit probably not self-employment income because he probably isn't in the "business" of ripping people off and taking their money. Either way, he would have a rather substantial amount to report (I don't know the total amount he received but I would imagine it is probably in the tens of thousands of dollars) because any money received illegally, be it from selling drugs or from stealing or from defrauding people, is required by IRS law to be reported which is how they often nab drug dealers and other people involved in criminal activity. The IRS usually does a reverse audit in these situations, where they say you reported maybe $20,000 of income on your tax return but you have a house that is paid for and multiple cars, so prove to us how you are able to pay for those things and if you can't prove it, they nail you with tax evasion.
From my understanding, all Ivan has to do is put it on his tax forms as income. If he claims it and pays taxes, he won't get in any trouble. He should claim his gb fees too, but that's up to him. From my understanding, if a drug dealer records his income on his tax form and pays taxes, the IRS couldn't care less. So going there won't really do anything. GMK pays taxes and individual GB leaders should as well on any income - besides that, GB's seem pretty legitimate (Source: my tax understanding and speculation).
This is true in a sense, but I think about 35 years ago the IRS nailed a big drug dealer for tax evasion and he ended up getting away with it because he went back and amended all of the years in question by treating his drug dealing business as a legitimate business. This means he wrote off trips to pick up the drugs, his costs for purchasing the drugs, and everything else associated with his illegal business then promptly paid the taxes owed. Well, the IRS had to let it slide because of how he went about doing it, but they quickly added something to the tax code called IRC 280E that says:
"No deduction or credit shall be allowed for any amount paid or incurred during the taxable year in carrying on any trade or business if such trade or business (or the activities which comprise such trade or business) consists of trafficking in controlled substances (within the meaning of schedule I and II of the Controlled Substances Act) which is prohibited by Federal law or the law of any State in which such trade or business is conducted."This is pretty much in regards to controlled substances so not all illegal activity is covered, but it has became a huge hurdle for places that have legalized marijuana now because they can only deduct their material costs but nothing else, so these companies are having to get very creative with their taxes to avoid paying taxes out the ass.
For other types of illegal activity, such as fraud and theft, there have been rules in place for many years preventing deductions of costs related to that illegal activity, but if you claim the whole amount you stole or defrauded people for, the IRS won't care because they just enforce tax laws and their powers end there.
