You should do your own little study of prices for single stamps not only offered but actually sold on
ebay. Those prices are probably in the range of wholesale to low retail prices in most cases. You will find the early Queen Victoria issues go for a fairly low percentage of catalog value.
One factor you did not mention is whether the used have pen cancels or not. High values also have pen cancels combined with normal postmarks and also pen cancels removed with a fake postmark added. Those usually are at 10% of catalog, or less.