There has to be some overhead in the need to switch from GPU to GPU, so I'd still think that one more powerful card would be a better choice, not to mention the fact that unless you get some kind of fanstastic deal, a single card will still be cheaper especially if you factor in the need for a very large power supply. But obviously, it's your money, and you can spend it however you want.
Personally, I wouldn't buy a graphics card from ebay even if it meant having to use integrated graphics for a while. There are WAY too many abused cards on there, and fakes. The majority of cards for sale on Ebay right now were mining cards that were either run 24/7 for months and months on end, and rode hard, or have had their BIOS altered for mining. Sometimes you can get them back to the original gaming firmware, but not always, and usually any card that was altered has been used like a downtown girl anyhow and isn't worth any investment, much less what they are asking for them. Some get lucky, most do not. Obviously using a reputable seller raises the chances of success, but then again, there are a growing number of hacked Ebay accounts selling crap as well.