redirecting the domain to the main site will help my site to rank my site on top of google that's why redirecting domain till getting sold is a good decision or bad?
That's confusing from the buyer's standpoint. I would contact you only if I feel that this domain is 100% of what I need, and nothing comes close to it... In case of alternatives I'd keep browsing, assuming that you need this name (since there is no visible indication that it might be for sale).
I wouldn't redirect if your main goal is to sell it. However, if your goal is a good link and you believe in traffic and SEO rather than selling - then sure, go for it.
It may be a positive for you in negotiations as the domain is in use. But the flip side is you may need a more motivated buyer as it might not be obvious the name is for sale.
But if it’s a good name that someone wants it won’t make a difference.