Okay here's the deal (at least in the UK, but I am pretty sure it will be the same in the US):
1. If the goods the guy sent you were the goods that you contracted to buy, but they are actually worth 10 times what you paid for them, then they are yours. Two people can agree to buy or sell something at a value other than market value. In the absence of any mitigating circumstance (ie undue influence etc) then its just the nature of the deal and you scored well on a smart buy.
2. If the goods the guy sent you were not the goods you contracted to buy (ie the vendor has made a mistake), then you are not entitled to keep the goods. There was no legal intention on his part to sell the item he sent you to you for the price you paid. Therefore there was no contract concluded for the item you received and you do not own it. Now, there are some legal defences to having to return them (for example if you relied on the goods being correct to your detriment), but given you know that it is not the goods you purchased (and you have written about it on the internet), if you were sued, you would probably lose and the goods would have to be returned. Now, the cause of action for this will last for, around 6 years from the date he sent you the goods. Which means, any time in the next 6 years, the seller could make a legal claim for you to return them. You also would have a claim against the seller to send you the goods that you contracted to buy.
Now, whether or not you care that you could be sued for there return is really a call for you, but technically the thing you have is not yours and your obligation is to return it.