HN members please advice on what you think of the idea and any suggestions.
IMO though I wouldn't be inclined to use it. First, if I buy something nice, new, or expensive and I want my friends to know about it, I will either put it on Facebook or tell them over the phone or in person. I know everyone I friend on Facebook personally, I have met all of them in person and talk to them at least once a month on average in some setting either via Facebook, email, in person, or on the phone. With that in mind though, of the over 500+ friends I have there, I would only share a purchase with maybe 20-30 of them at most depending on the purchase. I have lists setup on Facebook for all my friends, broken down into groups such as College, Work, Family, etc... From there when I post things to my wall I direct those posts only to those lists, meaning that others can't see the post. So I can control who sees what and knows what about things I do at a very granular level. Even though the average person probably doesn't have this level of granularity setup on Facebook, the thought process behind sharing is probably the same to some extent.
As for competition, I think there are several avenues you may not have considered. First, social shopping sites are starting to popup everywhere. Fancy (http://www.thefancy.com) is one that I can think of off the top of my head. There is also a site that lets women, and I think men, rate each others clothing choice.
Now, I will say that the concept of social shopping is powerful and if you can tap into the market with something unique and powerful you may have your hands on something. I for one know that I usually look to friends when deciding on a major purchase. If you could somehow incorporate a deals functionality, maybe like a Groupon into it, where if you can convince a bunch of friends to buy something you work out a discount with the store, that could potentially take off.
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