It usually takes much time to buy things with them because they never buy the first thing they see.

跟他们一起买东西要花费很多时间,因为他们从来不买他们看的第一件东西（意思说,喜欢“逛“）they never buy the first thing they see。为什么they see前没有不定式to呢？为什么要有to呢？加也应该加what或者that啊也就是说they see是做名词讲是吗？because they never buy the first thing they see.单看原因的后半句，主谓宾,整体框架是they buy thing，这里they see是对thing宾语的一个补充，修饰宾语即定语从句。