There is nothing wrong with guest posting.
Ranked dozens of sites successfully by buying sponsored posts and I have published them myself as well on some of my sites.
I would advise against link exchanges though, cross-linking sites may get you in trouble.
when you evaluating sites for buying sponsored posts look at these metrics:
- DA / PA (Domain and Page Authority, the bigger the better)
- relevancy is it a site within your niche
- check for actual traffic - SEMRUSH is your friend - you can also see if the site has organic traffic and what keywords are bringing the hits
- look at existing content on the site - does it look spammy, is this a site you wouldn't mind browsing actually?
If these points check out then I normally consider a guest post purchase if the price is right. Savvy tip: always ask for bulk pricing
hope this helps