The normal rule of thumb with hatchlings is a feeding cycle of at least 3 or 4 feeds and at least 1 shed during the same time frame. It's imperative the shop gets the snakes on a regular feeding cycle to ensure they stay healthy once they leave the shop. One feed isn't enough in my opinion to guarentee your new pet will continue to feed regularly once you get it home.
As for buying from pet stores vs. expos, it's really a matter of choice. While alot of the big chain pet stores don't have the knowledge or experience to sell reptiles, many of the smaller shops, especially those that specialize in reptiles do sell quality pets to their customers and will stand behind them if something goes wrong. If you decide to buy from this store, make sure you find out what guarentees come with your pet and that they are decent guarentees at that. Ask who they use for a vet to ensure their health and potential treatment for ailments. Any reputable petshop should not hesitate to give their vet's info and contact information to you should you request it. If they don't, stay away and go somewhere else. There's got to be a reason why they won't disclose who they use to ensure their pets are healthy and ready for resale. Pay particular attention to the state of the shop as well. Is it clean? Does it smell dirty? What do the tanks look like they are keeping their pets in? Is there rotting food, old feces or other uncleanliness in the tanks? These are all great indications about how they care for their pets while they are waiting to be sold. If the tanks are dirty or they are using unsuitbale substrate or there are other indications these animals aren't being taken care of properly, there is a VERY good chance you will end up with a snake that will, at the minimum, need veterinary care and will probably be a headache to get to feed decent and aclimate to it's new surroundings. For a first snake, you sure don't need those headaches right from the start, trust me.
Let us know what you decide and how you make out!