Is there a reason not to?
Vetting. Privacy. Billing. Service. Coordination of sales.
None of that is difficult. The hardest part is the marketing, and if ebay can do that for us...
It's not difficult, but it is time consuming and a hassle. I've done private sales for a few AUs and sales through both Old Company and New Company. As ARS says, private sales are more profitable but I can confirm - having done things both ways - that it is more hassle and time and work.
Personally I'm lazy so I just use one of the companies. For someone with more energy or ambition, private sales or eBay sales may make more sense.
I also think you might be underestimating the value of vetting and the stress of having to do that yourself and the infrastructure to do it well. But that's just my opinion.
whats the typical payout increase from selling privately, 2x? 4x?
Well, it's obviously up to each individual to decide how much they want to ask to be paid. I've only done a handful, and I think I doubled what the TL companies paid me and then rounded up a bit, so maybe 225% to 250%. For me, the hassle is the same regardless of the commission, so I tended to have a higher multiple on lower commission cards. So if the TL company would have paid me $75, I'd offer it to an individual at $200, but if the TL company would have paid me $200, I'd offer it to an individual at $400.
Regarding MinABQ's comment, I disagree with the assumption but I do agree that the risk is definitely higher doing it this way. I have only done private sales to people I have at least some connection with; I will not just do business with random people. Anyone considering this should certainly do whatever vetting they think is adequate. And there is a definite risk that the vetting that one thinks is sufficient may not be. But I think there is room for private sales where the seller gets more and the buyer pays less.
Another thing to consider - at least for me - is that I typically sell slots on cards privately that are already on offer at the TL company, which means I need to coordinate between the two channels. This loses some efficiency because you have to block and unblock a slot at the TL company and is obviously a bit more work because of that.
Obviously all of this is a judgment call, and anyone contemplating this should research it, understand what they're doing, and evaluate all of the risks and effort involved. And obviously I and ARS and anyone else on this board are not liable for whatever you choose to do.