There's no way to sell cards is there?

No one knows.

Last time cards rotated out they did a beta wipe and gave everyone gems to replace their real money gems. There's not currently any information on what will happen next time.

rumours say that they will make an arena only format where you can also play the sets that aren't in standard anymore.

Bingo.

Wizards hasn't published the plan for Arena rotations yet. Once they do, we'll know. Until then, it's all just speculation.

My guess: I think they’ll opt to wipe those cards with no benefit to the owners. This will be met with significant anger from the community, which will result in some minor “gift.” They will then promise to create a new modern-esque mode in time for the next wipe. By that time they will have abandoned Arena entirely, in favor of a new-new game.

Jk no idea

I think it's been heavily implied that we're gonna get some sort of "new modern" with all the Arena sets once rotation hits again, just depends on how far back they want to go (KLD, or Origins would be my guess)

KLD is what I've always heard. I've heard that from a while back also. I guess in October we will get Modern back to then. They are only going as far back in Modern as they have coded already. Makes good business sense. If this becomes a big enough money maker as I think it will I believe that not only will we will see it ported to consoles as was stated at the very beginning and then slyly disappeared but we will also be seeing Vintage and Legacy modes.
It will just take them a provable, solid monetary base and projected outlook to do the extra work.
If we make this game a popular, profitable game for them I think we will get these things and MTGO might just see its demise finally.
BUT!!!!
There has to be a real economy that duplicates MTGO in single cards though or there needs to be a complete buyout of all digital owners.
It can either be a mess or really good. At least I have to give them this... they have evolved and matured a little digitally so Arena just might be the new thing.
Either that or Arena is just a learning tool for them to makeover MTGO.
Hard to tell what they are thinking.
I digress.
Yes, I think we will get a Modern mode going back to KLD in October.

Shadows and dust, Maximus

A modern mode releasing all of the sets back through Kaladesh all at once would be a terrible idea. If they wanted to do that, they should have implemented it when people actually had those cards, rather than trying to reintroduce them later.

This is a Free To Play game. You are going to have major population problems if people suddenly need to go get 6+ brand new expansions all at once to be able to compete in a new mode.

Being absolutely frank, there's nothing about Arena that convinces me that they are going to dramatically expand their player base far beyond where it currently is.

Lets be real now. There's very little chance of a "modern" que that's free. More likely it'll be 1500 gems if you want to play with your rotated cards.

Devs already confirmed we will keep old cards and there will be an Arena-modern format, which will indeed start with next rotation in fall. What sets might be included I'm not sure but hopefully everything they encoded so far.

They're starting a new eternal format beginning with the cards currently available in Arena?

I don't think this is true, as much as Arena does have its criticisms i'd say it's doing the opposite - it's actually exploding in playerbase numbers

90k users on the arena Reddit, and it hit 50k users only about 3 months ago, so has nearly doubled since then, and it's clearly nearly all new players judging by the complaints about control, mana screw ect haha

And this is before the big push into e-sports with Arena, I can't see the playerbase doing anything but go up for at least a year as long as they don't majorly screw up

I'm not going to comment on the rest of it, because arm-chair economics is only my forte on Sundays and Tuesdays, the rest of the week being dedicated to armchair astrophysics and psychology, but this particular statement strikes me as quite odd without further context.

I have a bunch of Friends who have played MTG since 2008. 2/3's of them complain about control and 100% of them still complain about mana screw. There was also a recent MTG post on Reddit where someone's Control opponent handed them a coloring book lol. The caption read "Nice control players give their opponents something to do during the match."

That sentence may not have been the serious part of my comment

You will get 1 crystal for each rare, 2 crystals for each mythic and you'll end with a number of crystals ending with 9, triggering your OCD till the end of Arena.

I was/am under the impression that no one on here who has been around for more than a month wants to have a serious discussion with me on anything.

With the exception of thedevilwuster. He likes to discuss food.