NEW YORK ( TheStreet) -- After 17 years of marriage, my wife and I have a tendency to tell each other the same things more than once. The following conversation repeats itself often in my home after receiving a package.My wife "Where did you get that?" Me "I ordered it on eBay ( EBAY). My wife "I don't think anyone buys stuff there anymore, why didn't you order it from Amazon ( AMZN)?" My response is usually the same, "I checked both and eBay had the better price". She is right about one thing; a lot of sales have shifted to Amazon from other Web sites. eBay isn't always the lowest cost on any given item so if you're looking for the best price it's worth a few extra mouse clicks to compare. From a user's vantage point, eBay and Amazon appear almost parallel in their sales approach. You can order from many merchants offering tens of thousands of almost every product imaginable, and have one checkout to finish the transaction, even if ordering from several merchants at the same time. eBay and Amazon aren't the only two that allow third parties to sell within their system. If you visit Sears.com ( SHLD) you may notice third party merchants offering products also. I think that's a brilliant move by Sears due to their already massive nationwide retail store footprint. Amazon is already testing retail pickup lockers in various locations. So far, the results are mixed. Staples ( SPLS) and RadioShack ( RSH) decided to pull the plug on their stores for Amazon pickup lockers. I don't know what the terms of the deal were, but Amazon must not have given much if RadioShack pulled the plug considering they are on the ropes. Not to be outdone, eBay is testing "Click & Collect" in the UK. eBay shoppers continue to buy products on eBay, and instead of shipping the product to their home, they pick it up at a selected retail location. The concept appears much the same as Amazon's. There is one noticeable difference that investors should keep a keen eye on. The terms of the retail location agreements could impact the collection of sales tax. It's reasonable to believe that if Amazon places a locker location inside a state without an otherwise physical location, that may create the nexus needed to allow the location's state to demand sales tax.