If I buy an iPhone 6 in Germany, will it work in the U.S.? I know this works the other way around because I have my old 5 here in Germany right now and removing my U.S. sim card and putting in a pre-paid German sim card worked just fine.

I'm due for an upgrade and my phone is basically on its way out. I'm considering buying a new iPhone 6 here in Germany. When I'm back in the U.S., if I put my old U.S. (verizon) sim card in, will the phone work as usual?
As long as the phone you buy is factory unlocked (not a "no-contract" carrier phone), it should work. Same goes for phones bought in the United States: as long as it is unlocked, there shouldn't be a problem

One thing to keep in mind, is whether the models sold in Germany support LTE networks in the U.S. and on which carriers the phone can be used. It's most likely that the only major carriers you'll be able to use in the U.S. will be AT&amp;T, Verizon and T-Mobile, among a few smaller carrier. See this link from Apple that explained which models can be used on which carriers.