Building Material Recycler
Add a new building that is able to recycle building materials. It could be designed similar to a factory with question mark "?" item(s) that you can produce using building materials. The ingredients could be a mix of a minimum of 3-4 glass/brick/slabs per "?" item and the item(s) produced would be random building material(s). Each "?" item in the factory could require more and more building materials, and time to produce while you also get more random materials.

Instead of selling materials this could be a fun way to obtain some of the materials you need, and not ruin the value of them if they were able to be traded.

Material Balloons
Maybe as an event or new gift feature you could send 1 type of building material to friends instead of coins/cash. Players could choose which material or the game could have a specific material featured as gifts for that day/week. It could also be a random material given in a specially designed material balloon.