pixel_player45526023 Just a little clarification to alleviate any confusion (hopefully). Any New items (Equipment, Food, Decorations) that you can purchase from an Event with the Special Items you get from that Event will only be in the Shop for the duration of the Event plus one week. So after the Event is over and pulled from your game you have exactly 168 hours to use those Special Items to buy something in the Shop. The only way you will see those New items available for purchase in the Shop again (at least as the game is now) is 1) if the Event is reintroduced for Free again, and 2) if the Event becomes available to purchase for play in the Shop. Neither of those two things will happen with the Summer Builders Event, though, because it was a one off. If you do have any Special Items from an Event that could have been used to purchase New items from the Shop and that Event does get reintroduced for Free or becomes available for purchase, then any of those leftover Special Items can be used at that time to purchase those New items from the Shop.
Events usually also have some Food item you can collect and make a recipe from in your Kitchen. Any new recipe that shows up in your Kitchen from an Event will always be available to make in your Kitchen. So if you still have the ingredients to make any of those recipes, then you can even if the Event is long done with. If any of the Special Items from an Event can be used in one of those recipes, then you can always choose to use up the leftover ones that way if you want. The only option other than the ones previously mentioned is to do as @gamemaster suggests and wait for the guy who buys up leftover items like that to show up in your Camp. He does that every few months. Or you could just leave them in your inventory forever.