i work out of my home instead of maintaining a brick and mortar store somewhere. as others have noted, it is very hard to keep such a business running. problem is everyone knows that 1 computer geek they can call who will work cheaper than you can afford to. so my working from home allows me to be that computer geek everyone calls with problems and i can still work quickly and rather cheap.
as for parts inventory, i don't keep much of anything in stock as a rule. i do take advantage of big sales i find and will buy parts i know i will use such as psu's and hdd/ssd's. i then offer them to customers at a small profit that is still usually lower than current prices if i can.
for instance those corsair CX(m) psu's are normally about $60-70 but can often be had with rebates for $40 or less. i grab a handful and when needed i can offer it to a client for $50-60 and can show them the current price so they get a "deal". i don't buy anything as a wholesaler though and am not sure in the smaller quantities i buy it would even make a difference.
since most everything is custom i build, trying to buy 50x i5's at a discount would probably never pay off as i'd not use them quick enough before they were replaced with newer gen stuff. plus prices often drop and what i payed today might be more than they cost in 3 months. the client will know this as well usually and i'd end up taking a loss on the part. there is a reason most businesses buy in bulk with pre-built office pc's and not custom ones as well. they get decent warranties and so on that would be hard to match myself. so i gave up on a bulk office sales a long time ago. just not likely to happen.
most of the money i make is repair/replacement of parts and windows troubleshooting. i charge $100 over parts costs to fully build and get running a custom build. average repair is less than $50 and more than $75 for a full windows reinstall is a hard sell as well. quantity is the name of the game for me. but word of mouth works well once folks see you know your stuff and don't try to overcharge. so i stay plenty busy.