Is there an online marketplace to buy backlinks?

None that l am aware of, but any good kw tool should offer similar sites that may want to play ball with you, when contacted.

There are loads yes. But, essentially they are against Googles TOS so they generally don't advertise themselves or certainly not the sites links can be gained on.

Your best bet is to join relevant Facebook groups and ask for XXX niche link opprtunities then you will be inundated with everyone trying to sell you links. But only buy the best ones or you can seriously damage your website.

A few ideas erp...from a guy who's received thousands of requests like yours over a decade

1 - Pay for a sponsored post on high DA sites. Expect to pay $500 to $1000 or more for a valued backlink on these blogs. Some are No Follow - you pay for the privilege of aligning with these brands built over a decade or more. Others are Do Follow.

2 - Pay for a link on a high DA site. $150 or more per link. DF or NF depending on the blogger.

3 - Your site needs to be valuable and helpful for reputable sites to link in because nobody will sell out their reputation and years of hard work for $500 or $2000 dollars.

4 - You need to impress these bloggers to even grab their attention; we only pay attention and read emails from generous, mindful folks. I block all generic emails with poor grammar to filter out hundreds of such emails and to save hours of time. Learn the blogger's name. Publish a few genuine comments on their blog. Retweet their posts. Don't come in cold, like a stranger. Come in warm, like a friend. Stand out. Money does not sway these bloggers. Friends do.

A few steps to point ya in the right direction buddy.

Ryan

There are many providers on Facebook groups about backlinks. It's pretty hard to find them on Google because it's violated Google TOS.So, just search on Facebook something like guest posting marketplace, backlink marketplace, etc.

This can be done through manual search on Google/Yahoo/Bing. There are LOTS of websites under any niche online and they can be found my simply searching online. You can email them about your proposal and most of them will respond.