My guess is your website metadata is provided by Amazon so your primary "buy from" link must be from them. But I do believe you can list other vendors as long as Amazon remains your top choice.
Would you consider adding a "buy from" link to www.indiebound.org
? They have an affiliate program, you are supporting local economies and those who support local initiatives would be more inclined to make purchases through you (ie me and most of my friends).
I won't go into more "whys" of this--a google search will enlighten those who are curious about what's going on..