On eBay it's possible to set the maximum number of units that a particular buyer can purchase. Why would you want to do this? Well imagine if you had a great sale price on packs of batteries but then a single person bought all your stock to onsell themselves... that wouldn't be a great experience for the rest of your customers!
There are two ways to limit a buyer's purchasing ability. Firstly you can set a hard limit on a particular product that means that a buyer can only buy so many for the lifetime of that product. This is set in Settings > Feeds > General in the section labelled "Maximum quantity per buyer". Note that this setting is applied across all products in your feed.
If this isn't what you want to do, there are eBay settings that you can access in the eBay Seller Hub, under Site Preferences > Buyer Requirements. These settings also apply across all products, but allow you to block buyers that are below a certain feedback score (so potential bad customers), block buyers who have bought more than a certain quantity of items in the last 10 days (note that this is across all your items so be careful if you typically have large basket sizes), etc. More details are here: