After inflation, $649.00 in 1990 would cost $1260.00 in 2018.
So is $700 on eBay really that great now?
I guess, if you're not playing it anymore it's a good chunk of change, but eBay's gonna take their 10% cut, and sellers are supposed to pay tax on sales now in most states too. You might be better off selling it elsewhere.
It's hard to say if there will be the level of appreciation we have for classic games now in future years. My parents used to ride motorcycles, and for a long time prices kept going up, but now old bike prices are dropping like crazy, because young people don't give a shit. I'm sure kids that grow up playing games on streaming services in the cloud aren't going to give a fuck about our dusty plastic. Now days you can get most of the good NG games for $8 or less on Switch, and don't have to invest in any additional hardware.