Ricardo offers a (very) marginal better protection to the buyer/seller than the other sites, in exchange of am enormous 9% fee on the selling price.

Check with google on similar treadmills + the keyword ricardo.ch, to have an idea about the price they achieved in similar sales.

You may have some luck with facebook marketplace, it is quite intensively used.

More than 30kg via post is going to be extremely expensive, if possible at all.
You can however ship it in different packages (and it is still going to be extremely expensive, the bottom part is too long/large, so it does not fit in the standard sizes).

Can you tell me what your connection to the website is?
It's your first post, that usually rings alarm bells.

https://gratis-inserat.com

zombo.com