Sale Limits allow you to limit the number of times a particular file is sold as print/product/download from the Events and Galleries area.
To enable, add a keyword like "tx1_sale_limit:75" to the Keywords attribute on the Edit File page (where "75" is replaced with the number of sales to allow).
This feature is intended to help with selling limited edition prints, where you only have a certain limited quantity in inventory and you will not be getting any more. The number of remaining items is not displayed to the site visitor.
Whenever there is a sale of the file, the system automatically reduces the tx1_sale_limit:NN number by the quantity purchased. (This happens for any sale of the file, be it print, product or download — but in practice a limited edition print will only have one sale-item available for it.) When the sale limit is reached, the "Item Group" field on the Edit File page is toggled to "Do not sell this item" and you receive an email notifying you that the limit has been reached.
Warning on multiple simultaneous buyers
This feature does not protect against multiple buyers purchasing the last print at the exact same time. In that very rare situation, you should simply refund the losing buyers. This feature is really intended to help with automated inventory control for limited edition prints that have just a few sales per day.