The backlink profile of a recently-available domain is a double-edged sword. While some expired domains have been well cared for by their previous owners and have high-quality links from reputable websites, some contain quite questionable backlinks. The worst case scenario is that the domain was merely used as a link farm to strengthen a major project in a satellite domain network and has already been penalized by some search engines.
Perspective buyers are able to directly contact domain holders in cases where the desired domain is no longer available. Most registries openly publish the names and contact data of domain holders. Once this information is gained, buyers can get in touch with domain holders and make an offer for the name. Sales are also known to occur in instances where the original domain owner had no prior commercial ambitions
That said, my recommended budget for beginners would be $500+. Using this budget you could buy a bunch of high-potential $10 domain names, expired/dropped domain names or a mix of both. It’s very important to invest only what you can afford to lose and treat this as a side hustle till you get the momentum going. As you gain experience, industry expertise and some sales under your belt, you can then consider slowly growing your business into a full-time gig.