I’ve been using two M300 Dual printers for quite some time now. I wish you added the following spare parts to the store:
Heater & thermocouple assembly. Why? Because there are several things that can go wrong (including massive print detachment, material leaks) which can damage them or make them stuck inside the hotend’s aluminum cube.
The aluminum cube in which heater & thermocouple sit. Why? Because these cubes can be easily damaged when changing the nozzles (thread) or disassembling/assembling the hotend. After several nozzle swaps leaks appear on either side even with careful maintenance and even with a new module it’s kind of hard to find the sweet spot between damaging the thread with too much torque and having leaks due to leaving gaps on either side.
The only way to get those is to buy a full hotend module, which in this case is double-triple the price of other modules for which at least the thermal parts are available separately and the cost of a single part failing among the ones mentioned above is a really small percentage of the full modules price.