It can be very difficult to distinguish between
an offer and an invitation to treat. Basically it depends primarily on
the intention with which a statement is made.
Definition
of an invitation to treat
An invitation to treat is said to be a statement made by one person asking the
other to make the first person an offer.
An invitation to treat is sometimes described as 'an offer to make an offer'.
This is not a very helpful way of describing an invitation to treat.
If a proposition is made by one person with the intention that
if the other party accepts that proposition there will then be
a contract between them, then that proposition is an offer.
If a proposition is made by one person with the intention that
if the other party accepts that proposition there will not be
a contract between them at that stage, then that proposition is
an invitation to treat.