We have no built-in way of automating that at our end. However, you could set up a series of discount codes in advance and manually issue those to buyers that you notice are repeat customers. This help page explains our discount settings:
http://www.e-junkie.com/ej/help.discounts.htm
To find repeat buyers, you could download a copy of your Transaction Log to open in a spreadsheet program like Excel, and then sort rows by the Buyer Email column:
http://www.e-junkie.com/ej/help.transaction-log.htm
Note that the downloaded copy of the log would have a separate line for each item ordered, so if you get a lot of multi-item orders, you may want to do a secondary sort by Transaction ID to group the lines for each order together.
If you want to get extra sophisticated, you could use our custom/third-party Integration feature to have us submit the order details to a custom script at your end, which could then do whatever you wish with that data, such as counting sales to each buyer and emailing them a discount code when they've reached a certain threshold:
http://www.e-junkie.com/ej/help.integration.htm
