In order to maintain ownership of a domain, you must renew it yearly. If you don’t, it “expires”. An expired domain goes through several stages before being released again for general registration. The process differs slightly from one registrar to another but it’s more or less the same. GoDaddy explains what happens to domains that expire with them here.
Domain name sellers are advised to use to list their domain name. The real advantage with is the mandatory escrow service that comes with every transaction. The site charges the seller a commission of 15% per transaction or USD 50, whichever is higher, for domain names featured on the site. Sellers who do not list with but would like to use their escrow service will have to pay the minimum transaction fee of USD 50, or 3% of the transaction, whichever is higher.
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.