Do you have anything to buy?

Do you have anything which you will buy?
=Do you have anything to buy?
=你有东西要买吗?

Do you have anything to be bought?
=你有东西要让别人买吗? (somebody else might buy it)

have something/anything 解释中带有“有”的，用 to do 比如“有什么要买的吗？有零头能找开吗？”等等
have something/anything 以物为中心而不是以人为中心时，用to be done,一般用的没有to do 多

have something to buy: something 是 to buy 的逻辑宾语，不一定非得用过去式才能表达逻辑宾语的意思。英语中常有这样的例子：

Don't you have better things to do?

这里的动词不定式就是用来做前边名词定语的，在逻辑上是动宾的关系。

to do 是将要去做的，而to be done 是已经做了的

不知道。反正用起来就这样，具体不知道怎么说清楚。