They say that if you buy a domain, it'd be better if you put a dash on yourdomain like this-is-my-domain.com. Other people say: "That's an obsolete tip." Do you still believe in dash?
Depends - if you are developing a site and a second domain is not a significant expense you could buy both, put a 301 redirect on the non-hyphen to the hyphen and build on the hyphenated domain. Search engines can read names with a hyphen better - but only one, more than one hyphen is considered spammy. Only do this if you also get the no-hyphen variety, or you will loose traffic to that. If you are buying the domain to sell it, in most cases forget hyphens, they sell for much less than non-hyphens.
For SEO there is almost no difference but for human remembering/ potential marketing without the hyphen is better.... see for youself, say the domain "I am an accountant dot com" -v- "I dash am dash an dash accountant dot com"
No, that's stupid. Makes the domain totally worthless with 3 hyphens! I'd rather just get the .net without the hyphen or think of another domain.
In some cases hyphens can help make your domains more readable. domainnamesseller.com domain-names-seller.com
Without a dash is better no doubt. It will be easier for customers to type in your site directly from the URL and will also lead to good search engine placement (as well as the alternative hyphens version)
I just read lately that the title of the article is the most important in gaining high SERP. I've seen sites whose domains are badly written, yet they top in search engine results. Search engines are getting smarter today than in the past?