I personally am always disappointed by how much a seller of used miniatures thinks he should get. The conversations verge on the surreal sometimes.
Assuming your miniature pack is available at a regular retailer, there is no incentive for the buyer to go to you unless the price is lower.
Online, most miniatures are available from some legitimate online retailer for a discount. So you need to think of that as the actual price for your purposes.
And irrational as it seems, personally when buying used miniatures I add the shipping to the cost, whereas when I buy new miniatures, I do not. I have a feeling I am not unique.
And of course there are other people selling similar used miniatures whom you are competing against, so if you want to move yours quickly, you need to offer an incentive -- an additional discount. Now if you can be patient, and don't mind waiting, you don't have to do that.
That said there can be one positive price factor working for a seller, and that is if the figure has been in production a long time, and its price has been raised since you bought it, you should be comparing against the current retail price, not the original purchase price.
So to answer your question, I would say the highest any seller of used miniatures in new condition should expect would be 70% of the current retail price online, and maybe 80% at a convention flea market. But he should expect to take weeks or months to sell at that price; if he wants a quick sale, 50% online / 60% in person is probably the highest, and that's for high demand items.
That's for in-production figures. If they are out of production, a collectors mentality emerges. But most items are actually out there in fair quantity, so the retail price for a comparable miniature is probably what you would expect to get. Only truly scarce items would go for more.