I've been doing it for 12 years, full time for the last 6. You can't make any money at it and I wouldn't try. But tell you what, let me know what parts you have and I'll give you a decent price for them, especially minifigs. Seriously, it's just like running a car or motorcycle parts house. As stated above, you need good organization skills as well as familiarity with the parts. If you can look at a pair of legs and know that they belong to the Zeb minifig, it'll save you a ton of time over trying to look it up to discover what they are. I also determined early on that I don't want some customers. I don't want the customers that order 240 different parts, 1 of each. So I set a modest lot average minimum of $.90. I also have an $8 minimum order. True, I lose some small orders but, those are not my customers and I do not want them. I value my time more. You just have to decide what type of store you want to run.
One other word of advice. Don't contribute to the race-to-the-bottom on items. If 20 people have a part and the cheapest is $.20, don't automatically go to $.19. It makes us all poor. Some store owners never seem to realize that, you can sell that same part for $.28 IF you have the other 40 parts that the buyer needs in stock. I'll gladly pay a higher price for a part NOT to have to place yet another order, with shipping.
Good luck to you and don't hesitate to ask questions here.