I’ve had a little comparison and I think you should do the same.

I chose your last book, Leonardo’s machines.  You have it listed at $90 with free postage. 
As you can see by my pic, the same book is also widely available on eBay for roughly, a third of the price.

Please tell me why I should pay triple $$ for the same book? 

I think you need to sit and do a bit more realistic research.