Always ask for samples. Look for relevancy more than metrics. If the seller can only offer general websites for your niche, don't do it.
The marketplace is a bit chaotic with all the sellers with massive infographics. Be aware, there are many guestposts farms. The metrics (and even the traffic) look good but those sites build on expired domains are ready to explode after 2 years of massive guestposting.
Don't get impressed by huge discounts. It always means the service is extremely overpriced.
I also suspect there are some falses reviews to boost the sales.
The reaction score here is a good metric to see if the seller is more than just a link reseller. The only good service I could recommend so far is made by a guy with very high reaction score here. No surprise I guess.
The more I test and buy here, the more I think having your own lists of direct contacts is the best thing to save some money and to do some more precise SEO. Buying quality lists and making your own lists (I am doing it for the good gigs on Fiverr) are the most powerful tools for your SEO adventure in the long term.
I think it is better to work directly with the website owner for better quality inside the post. I've seen a bit too many barely readable articles with links which look like akward inside the article. You are losing tons of relevancy. Metrics are a bit overused here.