Avalon Acoustics Ascent MKii value?

Hello

I found someone who is selling these Avalon Acoustics Acent MKii speakers.  Giant, grey/silver.  And I simply have no idea where to start in terms of making hi an offer that is fair to him and leaves room for me to easily sell them for a profit.  Any tips or any resources where I can find out more information about valuing these would be greatly appreciated!  Thanks!