shall i buy .com or .net? My desire .com domain is for sale for huge price. If i buy .net domain would it effect the traffic of web?

It depends on how your going about marketing yourself. It will have no effect on organic SEO, however if you are marketing via TV or radio where people have to remember and then go to the computer and type it yes most people will falsely remember .com and not .net.

The tradeoffs could be minimal. If your brand relies heavily on branding go for the .com. If its all SEO and people clicking links and not having to type the domain in themselves you can go with .net.

There is a premium if you can get the domain name. If purchasing the domain name is going eat into your marketing budget, hold off the .com name for now. Once you are successful you can always go back buy it. It might be more expensive, but you can afford more once you have proven your business. You can also look into other extensions like .io and the similar ones. The domain extension is only small part of your overall marketing mix.

I would recommend getting the .com and .net (if possible) for your domain. Given that someone already owns the .com version of your desired domain, it will cost you a lot more on SEO and brand building to ensure your users go to the right account. Not worth it in my opinion and your time might be better spent on finding an alternate domain name where you can get both the .com (and .net if needed) at a lower price. Hope this helps!

If brand is hugely imporant to the business you are trying to build and the .com domain is already in the same industry as you are trying to break into, you will likely struggle with SEO for the .net domain.
If your business is not competing in the same industry or niche as the .com then it's fine to buy the .net domain.
SEO and traffic have nothing to do with the Top Level Domain (TLD).

If you are outside of the US I wouldn't buy the .net, buying your local country domain will give you much better outcome in terms of SEO and local branding. But if you are within the US, then don't buy the .net, as there is so little use of that domain extension you'd better buying a .org at least people put value in that.

But if the .org won't match your brand/service/product then you can try something like the current hyped domain .io but these have their own issues, .io might even all be turned off/or have absurd pricing after the "war" over its ownership is finished. This happened with .tv everybody hyped it, nobody outside of the domain space ever understood it was a domain and now you'll almost never see a .tv domain, yet in the peak they where everywhere (like .io).

I don't advice going after the 100's of other domain extensions, as while they are fun to play with from a pure ad/sending traffic into them, it is really hard to get a user to understand how to type the email addresses or even what to tell others the site is. This coming from someone who own's 1,000's of domains, and I even own a really "unique" one: https://search.engine.optimization.training/ but again this is just used for sending people into from ads.

Consider .club too. Clubhouse's impact on the social media space are making this domain the hottest pies of internet real estate.

I would buy the .com and the .net. and have one redirect to the other. I would use the .com as my primary address, and then have .net redirect to it. Then you own both, and don't have the risk of someone buying the other domain and doing something malicious.