yes, $100 would include a generous tip for packing and driving to the post office - assuming frame and fork are not unusually heavy...
just tell the seller in advance that he MUST keep the box below 108" length+girth with girth being 2x width + 2xheight (some don't know...) if he wants to ship it via USPS. he may be able to find a frame or BMX box which should just be within this limit but the frame may not fit into it if it is large. then he'll have to custom build a box by cutting down a normal bike box and taping/glueing it together. needs to be done carefully for it to survive the long journey.
Carsten