What bike is better 2010 Trek FX 7.5 or 2015 Trek FX 7.2?
At the level of bike you are looking at (2010 Trek 7.5 FX), most of the components will be easily found at your local bike shop or online. At this level, most manufacturers will use standard components which are easily obtainable, mostly because using any non-standard components would end up costing more, as they would only be produced in low quantities. Also, because they use standard components, and many other bikes use the same components, there is a big market for replacement parts, which means that finding replacement parts should be easy and inexpensive.  Assuming the used bike is in good condition, you shouldn't be worried about being able to find replacement parts when the time comes. As you move up to more exotic/expensive bikes, you'll start to see more unique parts that may be harder or more expensive to replace on older models.