In One Word: Trust
First and foremost, will people be able to really trust Facebook with their credit card numbers? Partly due to Facebook's reputation for "oops, my bad" style privacy breaches as well as the recent high profile data thefts at a number of retailers, I think users may have a hard time turning over secure information for potential exposure and abuse.
Secondly, will users purchases be automatically shared as posts to their feeds? Or, at the least, have their retail experience shared as an "endorsement" for an advertiser? Or will this be opt-in only behavior?
Lastly (and maybe most importantly), will the marchandise be worth buying at all? Facebook has a reputation for just as many low quality advertisers as it now has higher quality ones. And, if you don't much of a profile or activity, they throw pretty much anything, adwise, at you, regardless of how irrelevant it may be.
I see a small minority finding it useful but an intrusion at worst, an annoyance at best for the majority.