I sell through other methods as well as on Ebay, & I constantly get offers at 25% of the selling price. These are always via email, but i don't waste my 2 seconds to give them a reply,  I just delete them as they are professional time-wasters. Or they say "what's your best price?" , so , if I am selling something for say, $100, I will tell them that MY best price would be around $800, but if I got $100 for it I would be happy. Leaves them confused, and they go away,  especially after not even seeing the item, or asking the right questions about it.