I think the idea is nice in theory, but in reality this does not work and I hope you understand it.
First, items rarity depends on hero level. Your proposal means that a high level player could arrange high level items for others and sell it to them. Lower players would get items that they should not have at their current trophy range. Okay, there could be some constraint that degrades the item to max level of what a player max can get. Even with that extra rule a team could search for a single member for an item he wants and sell it to him. Just make a wish list and others can search for you. As a side effect, less gems would be needed at grannies, more people simply means finding an item you want gets more easy. You have to invest gems and gold to get those items by yourself and maybe spending extra gems to get a chance at even better items at grannies.
Selling items also has some unwanted side effects. When we need gold, we already have the option to sell those items for 10% back to grannies. Flare doesn’t want teamwork for collecting loot for upgrades and also not for getting good items very easily without having to invest a lot of gold/gems.
Buildings are created by workers that you need to spend gems for to getting them available. Less workers simply means you need to wait longer before you can upgrade, more workers means possibility to build things in parallel, losing less time and speeding up the strength of offense and defense. Every upgrade of a tower costs gold and a lot of time to upgrade a building to max, plus several treasure rooms full of gold. What price do you think someone has to pay for a max tower, besides the time he saves that he otherwhise would need to do it by himself? When you start to upgrade a tower, the game offers you a high price in gems to complete just that level. To upgrade one skull tower to level 9, take the sum in gems to save waiting time plus the sum in gold needed to max it. I think that is a very high price in gems and gold. So this will not likely be implemented.
Items plus buildings need to be improved by individual players. You have to invest for those upgrades by spending gold and gems(cash) at grannies to get items and also invest a lot of time (or cash) to be able to upgrade towers, spells, troops and so on. If you want maxed towers and don’t want to wait, you can get them for spending tons of cash.
Top players probably invested lots of cash to get a maxed base. It would be unfair towards them if others could reach the same by teamwork. If you want better items, troops, towers or spells, either be prepared to invest a lot of time to get the gold by raids, spend cash for extra workers to reach the top and if not, then you need to be patient.