We really don't know, BUT historically iPhones are pretty good at holding value. Selling BEFORE an announcement is better than after, as they will start a rush to sell and of course at actual release the after market will be flooded with devices and prices will be at the low point. Selling a month before anticipated announcement is probably a good idea. You can then use an older phone until your iPhone 8 comes in.
But we really have no way of know what value you will get.
Tip to maximize value. Make sure to keep your original box, and accessories, keep the device in good shape by using a case and/or screen/body protector, and handle with care. Clean and wipe device with micro fibre cleaning cloth and/or alcohol prep pads. Avoid drops and spills. 3rd party cases and accessories can enhance your sale but do not add much value to sale price.