Here's why this is a bad idea. The rich will get richer and anyone else trying to sell anything will have even less of a chance than they do at the moment.
From what I understand you'd place your chest with the items you want to sell in your realm, correct? If so, here's what will happen.
1. All the rich players/groups with realms close to spawn will set up a bunch of chests that can be bought from 24/7. Due to the fact that they are at spawn they will get the most traffic and therefor the most customers. Shops farther than 500 blocks from spawn will get hardly any customers and even anything farther than 300 blocks will struggle.
2. The richer players/groups can often afford to set lower prices than the rest. Combining this with the fact that they're close to spawn means that everyone will shop at those 2 or 3 shops near spawn exclusively.
3. The fact that you no longer have to take part in the selling as a seller means you can sell everything at the same time without having to put any effort in. For example, most 'rich' players don't bother selling cheap, easilly obtainable items because it's not worth the time and effort. With the chests you would be able to sell everything while doing what you would normally do to earn credits. This takes the 'beginner items' market away from the 'beginners' and all small transactions would now also go towards the already rich players.
4. The fact that the chests will be available 24/7 means that anyone trying to sell anything during low traffic hours is also boned. Supply and demand has less influence on the price as there will almost always be supply. At the moment it's possible to sell things for a slighty higher than usual price when certain people aren't online.
Basically, this system is going to help the rich much more than anyone else. It's not going to make selling things easier for everyone. It's going to make selling things easier for people with realms close to spawn.
So go ahead and implement it. Personally, it would be a great boost to my income. I can finally sell all the junk I normally don't have time for to sell.
