Hmmm...well, I bought both the Equinoxe and Oxygene MFSL CD's a while ago sealed and all, but since I wanted to listen to them I removed the seal but kept the J-Card and it put on top of the booklet.
I never knew it was called a J-Card until I read this topic.
Regarding the value of the discs..if you have the J-Card and the original Flip-Up box, but the seal has been broken..that shouldn't be that much of an issue, does it?
I mean, come on..the MFSL discs scream to be played..they sound fantastic!
I's a shame that these discs are kept sealed.
[Nagging mode]IMHO all J-Cards are the same, so if you have another MFSL disc (for example Joshua Tree - U2) you can also put it with the JMJ discs and nobody sees the difference anyway.
Sealing systems are also not that expensive to buy and presto, a couple of hundred bucks can be earned!

[/Nagging mode]