How could I handle heavy load and consistency for product inventory in online retailers?
First off, taking a step back, do you really need to solve the inventory management problem on the front end?  Since you're selling large volumes of a relatively small set of products, it should be relatively easy to manage your inventory so that you are never out of stock or, if you are, it doesn't prevent you from fulfilling orders.  There is a great deal of literature and examples that deal with calculating safety stock which requires just a bit of statistics to follow.  It would make far more sense to me to focus your attention on giving the company the tools (if it doesn't already have them) to manage their inventory to prevent stock-out situations rather than trying to prevent them from happening in the sales portal.

That being said, I'm not quite sure that I follow your problem with the two scenarios you outline.  Even if the database performance was flawless, if you have only 1 of item A in stock and you can't sell an item if it's not in stock, then one of the two customers, by definition, one of the two potential customers is going to lose out.  If in the first scenario C2 is going to go away without buying anything if any of his 35 items are not in stock (which seems unlikely if he spent 20 minutes filling his cart), there is nothing you can do in the database to prevent that.  Your interface could potentially have some AJAX that alerts them while they're shopping that one of the items in their cart is out of stock much like StackExchange notifies you while you're entering an answer that someone else has entered an answer.  It's not at all clear to me, that telling C2 about the problem earlier is going to be beneficial-- if he's going to leave if he can't buy all 35 items in one transaction, he's going to leave no matter when you tell him that C1 bought the item.  Realistically, there is no way to design the system so as not to disappoint one of the two customers in that case.  

It might help if you can explain a bit more about why your application and your customers are so sensitive to stock-out situations.  Most customers and most retailers are relatively accustomed to the fact that sometimes after placing an order they get notified that the retailer isn't going to be able to fulfill the order as quickly as they had expected and are given the option to cancel that part of their order, the whole order (assuming the remaining items haven't shipped yet), or to wait for the item to come back in to stock.  Assuming that you do something to notify customers while they're browsing that inventory is relatively low (i.e. Amazon will tell you "N items in stock" if you're looking at an item for which they only have a handful of stock left), most customers are reasonably understanding 20 minutes when they get to the checkout and are told that the item is now out of stock since they knew in advance that they needed to order quickly.  And most retailers are comfortable that even if they run out of stock of most popular items, they can still satisfy more requests than they have inventory in hand because they undoubtedly have new inventory arriving in the next day or two or they can rush an order for new inventory.