I think acceptance of most major credit cards would be a given.
Yes, that's a good idea. I've seen it done but you have to be careful with volume. Additionally, it's a great way to alienate dedicated modellers who rightly or wrongly think they were taken advantage off (mostly those selling).
For a true hobby shop it's not necessary. You want to sell to people, not have walk in customers upsetting your mail order business. The question is: What is your priority, running a hobby shop or running a mail order business? Mixing both can be a problem unless you're a big outfit.