You can buy (almost) everything on the Internet.
这个句子没有问题。不过你把everything改为anything句子也是正确的,但意思有细微的差别。你之所以产生疑惑,原因是你忽略了everything是个总括词,而不是肯定词。所以无论是肯定语境还是非肯定语境都可以用everything的。例如:
He tried everything. 肯定句,肯定语境。
Did he tried everything? 疑问句,非肯定语境。
He didn't try everything, but he did try to talk with her. 否定句,非肯定语境。
I will do everything I can to help you. 肯定句,非肯定语境。
上述句子都是正确的句子。你误以为everything是肯定词,不能用于非肯定语境。你有二个误解。第一,你把everything误解为肯定词了。第二,你误以为肯定词就不能用于非肯定语境。看看下面三个句子:
Did you see anything there? 说话人不知道那里是否有东西,所以问句中用非肯定词。
Did you see something there? 说话人认为那里有东西,不知道听话人是否看到,所以问句中用肯定词。
Did you see everything there? 说话人问听话人是否看到了所有的东西,所以用总括词。
主人家桌子上有很多食物,主人对客人说:
You can try anything. 主人不知道客人是否想尝尝,所以用非肯定词。
You can try something. 主人觉得客人有想尝试的念头,所以用肯定词。
You can try everything. 主人想让客人每样食物都尝尝,所以用总括词。
你的问题我就不具体回答。你参照我这个解答自己对照理解即可。