The economics are simple - how much power do you use vs how much money you make per unit time. As a miner you're banking on the increase in difficulty correlating with an increase in price. Ethereum mining difficulty plummeted back in October, flatlined for a bit, and has been slowly on the rise since December.
There is limited resale value in the cards (depreciation etc), but even after PoS hits there will be enough mineable things that selling them off wouldn't be necessary. Love or hate it, you could use a service like Nicehash.
It becomes unprofitable if the difficulty spikes, but not the value, so your hardware takes a long time to mine little. Often this happens between new technology generations, like when ASIC miners made GPU miners unusable for Bitcoin.
Right now Ethereum is still quite profitable to mine. I have a bunch of GTX1060's with a combined hashrate of 100MH/s running on an EthOS system. That's enough to earn between 0.35-0.4 ETH a month at the moment. The power usage is around 300W with some twiddling - around £25-30 per month.
You also have to consider speculative value of the coin. Suppose you were mining 1ETH a month back in January '17. Your monthly payout (about $30) would have barely covered electricity, but now 1ETH a month is practically a minimum wage salary. You need to be honest with yourself why you're doing it - are you making money to spend today or are you holding?