There are hundreds of these "online stores" out there selling the exact same items you will likely be selling. You must identify your differentiator. Then you must understand that you will be competing with the big guys like PetCo whether you like it or not. That`s why your differentiator must be able to generate business.
Here`s what I suggest. This came to my head a few days ago while buying a computer monitor for the office (it`s so nice to be on a 22" instead of a 12".)... Anyway...
When buying this computer monitor the sales man said, "If you like that model, I`ll give you a little inside information. This model (Pointing at a very similar style, a few models down), is made by the same manufacturing line, and is essentially the same guts as the model you want. But in this situation the brand name is less recognized, so it will save you about $200."
So how does this relate to the pet store? I am sure there are third party solutions to common problems. For example, I have algae in my tank and I plan to buy the TOPS brand because they are tops in correcting my problem. My lack of knowledge just tells me to rely on the industry best. You may know of another product of a lesser known brand that beats, compare to, or is only slightly less effective. The savings to me, may be a couple dollars.
Also, as your store grows and you open a real facility, you can add the differentiator of public involvement. You could take a product and test it against another product. To inspire people to be involved, you could make a raffle, drawing, or some other game, resulting in a winner of the correct answer to the "stunt." Maybe the prize is a few gold fish or on big events as 10 gal tank set up. The walk through traffic might make the prize cost back. It all depends on your presentation and entertainment level. Maybe go to work that day in a clown suite?
Anyway, back to the online part... It will be very difficult to beat the others on price. Beat them by creating a resource online which people will access for information, only to learn you have the products available at a reasonable price compared to all the other suppliers out there.
No matter what. When starting an online business, the marketing and all that is more important than with a brick and mortar. With a brick and mortar, people driving by can see the business sign. Online, no one drives by unless you find a way to make a connection. So make one... Every day. Not just by way of the Internet. Talk to people. Tell them about why they will come to you instead of PetCo. It`s not because of your face. It`s because you offer something better. You save them money by not buying name brands. You inform them with your test and recommendations.
Vincent Wilcox (a.k.a. KRAKR)
My band: Letters Make Words