I use my credit card to spend $100 and receive 100 points which I sell for $1.60, I now received a $1.60 "rebate" on my $100 purchase.
If you buy a table for $100 and sell it for $200, the profit is most certainly taxable income.
Selling miles is a grey area. That doesn't mean the IRS can't sue and make you prove otherwise.
I spoke to an accountant, or two, who said the same, but said selling is a little Grey area(unlike straight up CB or using the points) the difference is when you MS, then it's all a profit, there any form of cash would be taxable, but points to use wouldn't be