Do the sites necessarily have to have traffic in similarweb semrush or ahrefs ?
no, but it helps if they have traffic because it's an indication that the owner cares about that site... usually...
What criteria should I use when checking them?
you need to make sure that they're not link farms (link farm = site that exists only to sell links, in the form of PBN link, guest posts, niche edits, etc.)
Of course, finding out whether or not these sites are link farms is not easy, and requires you having both SEO knowledge and flair. So, with this I can't help you, you need to develop the skills of assessing the quality of sites yourself
But, generally speaking, link farms are sites that meet any, most, or all of these criteria:
- they don't have traffic
- they don't rank in google
- their content is low quality and / or low value
- there is a link to low quality websites on almost every page they have
- their authority (DA, or DR, or whichever metric you want to use) is low (not always true, but sometimes it is)
- they usually look ugly, or like the owner doesn't care about them
- their OBLs (Outbound Links) are usually d0follow (because people buying those links WANT them to be d0follow so they can have link juice passing through into their sites)
If you find sites that tick most of these checkboxes chances are that these sites are link farms and google either already discards the links coming from these sites, or they will discard them soon (soon = anywhere from days to years, depending on your luck and on google's priorities and ability of spotting and flagging these sites)
If the sites have only DR 10 authority for example and very little traffic in ahrefs is good for a guest post ?
if the other checkboxes that I listed above are NOT ticked then yeah, the site is probably worth guest posting on. But again, it's not as simple as 1 +1 = 2, you need to check these things but also use your SEO experience and intuition to figure out whether the site is good or bad.
But the good news is that - even if the site is bad - google will usually not tank your site because you have links from it. Google will usually ignore these links, so it's only a matter of you wasting the money on low quality links that don't help you rank, that's the worst that can happen most of the time.
Now, if someone reports that site to google for selling links and google decides to look into this and / or manually penalize the site.... in this case your site might get de-ranked or even de-indexed, too. These things did happen in the past (and they probably still happen today, as well), but these cases are usually rare (you find maybe 1 such cases every few millions.... or maybe every 100s of 1000s of sites, so you're pretty safe in general)
