The Guilds of Ravnica pre-release is this weekend which means you don't have a lot of time to clean up your collection. Rotation happens Friday, October 5th when Guilds of Ravnica becomes legal. Many players will rush out to sell their rotating cards, but the truth is there is plenty to hang on to for a rainy day. Cards in the oldest two Standard blocks are at their all-time low and you will be unhappy if you sell them now.
Generally speaking, 6 months before rotation is when Standard cards start to decline. Cards from Kaladesh and Amonkhet block have been on a steady decrease since the release of Dominaria. If you had a lot of competitive cards you weren't planning on playing that would have been the most optimal time to sell. Now, it's far too late for a lot of them.
Lands Lands Lands
Investing in real estate (as some people like to say) is never a bad idea in Magic. Every deck plays lands and dual lands are especially important. The Kaladesh fastlands will probably never reach the prices of the Scars of Mirrodin fastlands due to their colors and availability but they are definitely going to be more expensive than they are now. Right now buylists are pretty abysmal at 50% or less of the retail price of the two best ones (Spirebluff Canal & Blooming Marsh). At this point I wouldn't bother selling them for $2-3 each because they can easily pick back up to $10-12 in two to three years. The Amonkhet cycling lands are probably not going to be as desirable because they always enter the battlefield tapped. That being said they are the perfect cards to return to Standard at some point in time just like the buddy lands currently. I would be surprised if selling them for less than $1 ever pays off. The minimum cost of rare dual lands has been around $2 for a long time. These are the kind of cards you can throw in a box and forget you own for a few years and then realize all the free money when you find the box again. It's also possible cards that are printed in the future could make them desirable in older formats. For example, Valakut makes the mountain subtype desirable and Life from the Loam makes cycling relevant.
Kaladesh Block
There are quite a few cards I think that are pretty easy to hang onto post rotation. Fatal Push and Walking Ballista show up all the time in Modern and Legacy. Their future seems bright and if you feel like you will ever want to play these cards they won't be cheaper than they are now. Outside of those, you may have some more casual cards that you didn't realize are quite as desirable as they are. Panharmonicon, Mechanized Production, Paradox Engine, Planar Bridge, Metallic Mimic, and Heroic Intervention are all cards that are more than bulk rares. While many of them are artifacts that could appear in a supplementary set, I think they're generally pretty safe to hold as things like new Commander decks can cause them to spike. Chandra, Torch of Defiance is the last card I want to mention because it's exceptionally cheap compared to its peak price. This card is arguably the best Chandra card ever printed and likely to be a candidate for long term growth. If Assassin's Trophy is truly the missing piece to making Jund great, it is possible that some decks eventually will turn to Chandra as another grindy card advantage engine. Either way I don't think selling Chandra right now is a good long term plan.
Amonkhet Block
Amonkhet is arguably a less powerful block than Kaladesh but there are a lot of casual focused cards that should continue to carry a good price tag. Rhonas the Indomitable, The Scarab God, and The Locust God are favorites in the Commander community. There is some precedence with the Theros Gods that suggest that even the worst ones will be valuable at some point. Even Heliod is worth a few dollars despite it easily being the worst of the mono-colored gods. Outside of the Gods, I think that Nicol Bolas, God-Pharaoh, Razaketh, the Foulblooded, Mirage Mirror, Torment of Hailfire, and Liliana, Death's Majesty are some of the cards I'm most looking forward to keeping. They're not high on the radar of a lot of Standard players so I would go so far as to suggest trading your pre-release Assassin's Trophy into these cards and watch your balance sheet for the next month. A lot of Standard players really underestimate the price and stability of casual cards.
Are there any other cards you're not sure if you should sell? Have other finance related questions? Feel free to leave a comment and listen to the MTGFinance podcast I'm a part of here.