"How much should I pay for one pound of Lego?"
If you are buying only one pound - Don't buy on eBay.
It is much better and cheaper to buy brand new Lego Building Sets
Is this worth $12 to $18?
Many sellers are getting $12 to $18 for only one pound of Lego.
At this price, it is much better to buy "Brand New Lego"
In the one pound bag of Lego, there were 205 pieces.
That's 6 to 9 cents for each piece of USED Lego on eBay.
What I think about used Lego
You can buyBRAND NEW Lego Creator Sets
or
Lego Bucketsand in these sets the cost is only 4 to 5 cents a piece.
How much should I pay for 10 or more pounds of Lego on eBay?
Try to buy it for less than $5 a pound
Buying Bulk Pounds of Lego on eBay : Lego Buying Guide