This is asked constantly and answered dishonestly almost as often. Here is the balanced version.
The case for sealed
- Supply only decreases. Every box opened is gone forever, and print runs end.
- No condition risk on the cards inside, and condition of the box itself is objective.
- Simplicity. One item, one price, no per-card grading judgement.
The risks people skip
- Reprints and reissues. A publisher deciding to reprint a set, or release an anniversary version, can undercut a box you paid a premium for.
- Box condition matters more than expected. Dents, crushed corners and shelf wear meaningfully reduce value, and storage is bulkier than singles.
- Authenticity and resealing. Resealed boxes are a real and growing problem at the higher end.
- It is illiquid. Selling a £900 box takes far longer than selling a £900 single, and the buyer pool is smaller.
- Opportunity cost. Money in a box is money not in anything else, for years.
If you do hold sealed
Store it flat, cool, dry and out of sunlight, and do not stack heavy items on it. Track what you paid, so you can judge the position honestly rather than by memory.
Sealed products with a live market price are catalogued here and can be tracked alongside your singles, with the total split so you can see how much of your position is sealed versus cards. Coverage is partial and the price data page says which products carry a price and which do not.