When I have a customer, I send them an email saying things like- Thanks for your purchase, I hope you enjoy the Pyrex dish! (or whatever the sold item is). And I say, if you'd like- I'll email you or keep you in mind when I get more Pyrex listed on the shop.
A lot of customers hit me up on that- I have a couple of previous customers who would like me to keep them updated on cameras and photography related things...
When I ship out peoples items, I write a little note saying thank you and telling them I appreciate their purchase and to check back often that I'll be posting more items that may be of interest to them in the following weeks.
I hope this helps :)