The price of all SILVER coins on ebay (and anywhere else), regardless of type, will go up when silver goes up and down when silver goes down. Sometimes slower than market price moves- but they eventually get there. Intrinsic value of slabs, proof, and collector silver always tracks the moves up or down but the % of margin or premium over spot may not move at the same rate. (Numismatic value is completely independent of silver value and can go any direction at any time...)
Silver at $22--- Regular bullion Eagles are 26-28, 2013 Slabbed PF70 eagles are $55-70, Roll of silver quarters $180ish
Silver at $40--- Regular bullion Eagles will be 44-48, 2013 Slabbed PF70 eagles will be $80- 110, Roll of quarters will be $380.
Timing a sell at a high or low is practically impossible- If you need $ now- sell'em. If not- take a gamble and hold on to them.
PEACE