I can't speak to whether PowerON is a scam or not, but they clearly do not offer a great value to potential device sellers. They quoted me $492.75 for my mid-2010 MacBook Pro. When they received it, they said the condition was poor and offered me $120 less, at $372.50. The things they pointed out were in fact there (minor scratches, for instance). But they were a) very minor and b) things you'd expect to find on any non-mint condition laptop. I told them during the quote-making process that the device was not in mint-condition, but they seem to set their quote with unrealistic expectations so that they can then point to those failings as reasons to pay you far less than the quote.
While PowerON was quite happy to mark down their quote for minor blemishes. They were just as happy to fail to properly price the device in the first place. Their quote process takes into account only the stock MacBook Pro model; they ignore any upgrades (both OEM and user-installed) in setting their quote because they don't ask about them. For instance, when I ordered my MBP I opted for the Apple-installed customizations of the high-res screen and the faster HDD. I'm sure PowerON noticed these things when they did their audit, and I imagine they were quite happy.
I asked them to send back my device. At first they offered my $25 more. Then when I still refused, they sent it back. It arrived in perfectly fine condition with no further damage inflicted by the PowerON team.
Next I sent the device to Powermax. They quoted me a slightly higher price than the PowerON original quote, and after they inspected the device they paid me the full amount quoted. I should have gone to them in the first place -- I've bought from them before, and they are super responsive over e-mail, and when you call, a real person picks up the phone. The downside with Powermax is that you get a Powermax credit instead of cash or an Apple credit, but if you're planning on buying a new Apple device, you can buy it through Powermax and take advantage of their trade in program.