I can say the following about this:
I have been a seller on fiverr for over a year, and I can give some advice to buyers.
1. Pay attention to the price. If you are offered 10 links for $10, this is complete crap.
2. Ask the seller for samples of work and describe what backlinks you want to get. In the correspondence, indicate the topic of the site and ask for an example of work with this topic.
3. If you have requirements for parameters, initially agree on this with the seller. Inflated parameters can increase the price. This is normal, you have to pay for quality. There are just such clients who want to get a thematic forum for $3, with incredible traffic parameters.
4. Specify all requirements in the brief.
5. This is probably one of the most important, always be in touch with the seller. For example, I can clarify some details, or the field of selection of sites, it is better to agree on them with the client, and then you will get high-quality work.