You might need to save an item for a specific buyer. You might see that someone is trying to find an item in the forum and you might have one that isn’t listed. If you list it in your store, another buyer might get it before the one you intended it for can buy it. Reserving it for that buyer keeps anyone else for getting it.
Find the item in My Inventory. You will see under the Tiered Pricing there is the field Reserved For. Enter the BL username of the buyer and save it. That is all that you need to do besides alerting the buyer that you reserved an item for them
Under the My BL tab and My Activity it lists in Selling if you have any lots that you have reserved in case you need to find it in case you need to unreserve it. I have only reserved an item twice so far so it doesn’t come up a lot but in case it does you can see how it is done.