Hi
@Giorgio Marandola and welcome to domain name investing and NPs!
As others have noted,the .fitness extension has limited registrations (about 10,000) and limited sales (2 in NameBio, one large but several years ago, 2 additional ones in $50 range at DNPric.es).
Prior lack of sales does not necessarily mean that a sale is not possible, sometimes even a large one. For example, prior to the late 2017 sale of vacation.rentals there had been only 5 sales in the extension (on NameBio at least) over 3 years, none of them above $2k. Then we have a sale at $500,300 (although none since then). This is certainly not the only case, in fact it is almost the norm in new extensions. For example in .loan not a single sale had been made by late 2016, and then the first sale was for $45,000 (auto.loan).
So really the question comes down to how good you(.)fitness is. The exact word you has sold for good amounts in multiple extensions including some new ones. For example you in club for $5000. In net the word sold for $23,000. The word you and fitness do fit together.
The fact that there is a shorter similar extension .fit has further made it difficult to sell in .fitness.
I would, if mine, try to get something high $$$ to very low $$$$ but be flexible if get to negotiation stage. I suspect some dedicated outbound is necessary to have a reasonable likelihood of sale.
The very best of luck with it.
Bob
ps If you want a totally numbers approach, in com about 65000 sales on NameBio per year. That means about 1 sale for every 2000 registered names per year. Over 4 years, about 1 sale per 500 registered names. In this extension 2 sales (over $100) in 4 years in about 8000 registrations (average over period), or about 1 in 4000, so only 1/8 as likely to sell as a comparable com, although small number stats means this is a very uncertain value statistically of course.