IOf you state on ebay that it is advertised elsewhere you risk having your item removed as well as ending up with 2 buyers and if you sell to the Gumtree buyer you will get an ebay defect. You will also greatly annoy a buyer who will almost certainly leave you a neg if it is ebay and will probably be sticking pins in an efigy if it is Gumtree. Remember too that Gumtree is owned by ebay.

If your items are pick up only then you would be better not using ebay at all, stick to Gumtree and facebook.