How big are yours? How big are the yellow bellies? How big is the lemon pastel or spider and are they male or female? Are you certain that you will be getting yellow bellies and not normals? How nice is the lemon pastel or spider?

These are important questions and depend on what you want to do in terms of breeding. With the yellow bellies you have a chance at Ivories, but you could work on a long term project of BELs and Super Black Pastels with what you are considering trading. In the end if they are good examples of their morph I would probably consider the trade if the lemon pastel or spider is female.