How can I allow front end users to add their own custom fields? We have a new project that is going to allow users to add their own nodes of a content type e.g. 'cars'.

There are some existing fields that they need to complete e.g. 'colour' and 'year'. We also need to allow them to add their own custom fields, which only they can use. So one user will add 'mileage' whereas another might want to add 'number of seats'. How would I go about allowing hundreds of users to do this without bloating the content type?

We will be building it in Drupal 7.

Thanks for any help.
Use Field Collection module.

Install Field Collection module 

Create a field collection field to a content type and set to unlimited.

Then create a fields with two text fields - "label" (for 'colour' and 'year') and "value" for that field collection

So when the node add form loads user will have two fields, Label and value with add more button there user can click and add another set of "label" and "value" pair.