I poked around a bit and found general information on Sebring and little on Cliff. Cliff as stated by KC, seems to bring up J. Dimmock ... Staffordshire... etc. but pretty sure that is not who made This platter. When time permits I may look a bit more. As for Sebring this is another Liverpool OH Comapny, in the city of Sebring OH, company closed in the 40's. It would like to see the hallmark on the Sebring Gravy Boat. This item has some promise of good value.
In general Stoneware, Earthenware, Ironstone items do have a moderate collector base. One thing also is most collectors of these items really care less about the hallmark and more about How the item looks in the deco. There is a serious collector world for Earthenware items, but they seek out items that are usually the cream. Another good thing about pieces like these is many are looking to finish or replace something in there set. If you can find the pattern name, you have a far better chance. Generally speaking good or great items (No Flaws) are worth $20.00 Most people if they want a item they are willing to pay $20.00, usually less but if it will help them finish their goal, that Jackson will slide easily from that wallet. Want more then you have to know the story, Company, History, or previous owners name you can tack on a few more bucks. The best item to break this Ceiling is that gravy bowl. If you dig enough you may be able to bring that into the $50.00.