请问对吗？ It is cheaper to buy old furniture then to have new one made .

可以。

then 是 than

It is cheaper to buy old furniture than to make new one.

错误:
It is cheaper to buy old furniture than to have new one made .
then 改为than

正确，没有语法上的错误。to buy old furniture 和to have new one made 是两个并列的成分