For many consumers (some reports say up to 70%), part of the buying cycle now includes reading product reviews before making a purchase and writing their own reviews after the purchase is made and they've had time to experience the product. These days, it's almost unheard of for any legitimate e-commerce site to offer products without allowing people to leave reviews and ratings (typically on a scale of 1 to 5). Pretty much any inventory-management or e-commerce system you choose will include the ability for people to comment on the products in your store, so implementation shouldn't be a problem.
The problem with many small store owners, quite frankly, comes down to ego and fear—which both go into overdrive when a negative review is left. Believe me, as a writer who has written nearly a dozen books, I'm well acquainted with the sting that comes from getting a negative review. Nothing is worse than going onto Amazon.com and seeing that someone has left one of my book a one- or two-star review. That sting can be even worse for a small business owner who amasses poor product reviews on their site. The knee-jerk reaction is often to delete poor reviews and only keep the positive ones.
Clearly, if every product in your store is getting poor ratings and negative comments, you might need to delete some of these just to stay viable as a company. (Although, if all of your products are getting poor reviews, you may want to consider whether the products are worth selling or if you maybe need to promote different inventory.) But in general, it's best to stay as transparent as possible, leaving the negative reviews on the site as well as positive ones. This will create a higher sense of trust among your shoppers, as a site full of only positive reviews seems a little shady. If possible, your best course of action is to reply to negative comments directly and publicly so that all consumers can see that you've taken the time to read the reviews and care enough to answer inquiries and possibly improve the product offerings.