IMO, if you have no intention of buying from a local retailer, leave them alone. While they'll likely be happy to share their expertise with you, it's because they're hoping you'll become a customer. In business, time is money, and it's unethical to waste someone's time, with the intention of buying online; a practice known as "showrooming". If you're going to DIY, you shouldn't need their help, anyway.
There are myriad sources of good information online, including forums like this one and companies such as Crutchfield which freely share their knowledge. You can take advantage of these resources without taking up a business person's time or adversely affecting their livelihood.
OTOH, if you do intend to buy locally, there's nothing wrong with speaking with more than one retailer to determine who offers the best products, service and pricing. That's fair competition and smart shopping.
Full disclosure, I DIY almost everything in my life, as I like to be as self-sufficient as possible. I don't bother local businesses unless I need their service and intend to hire them. I've also managed and owned retail businesses, so I have that perspective as well.