O buy from whoever has the part(s) I need in stock,or whoever can get them quickest if they don't..price is a factor too,if I'm not hard up for a part and I can save money by having the "cheapest" place order it in,I'll wait a day or two and pocket the savings..
I have yet to buy anything online,auto parts or otherwise..
No credit card,and I just don't trust doing transactions online..
Rock Auto does seem to have many "obsolete" parts ,but they also have sent the wrong parts to friends who have used them or substituted a different brand,some parts look like they were sitting 30 years in a warehouse too..but others have good luck with them..
I just do not want the hassles of dealing with over the internet "wrong parts" returns,or not getting the brand you wanted,or some petrified old stock that isn't really roadworthy..
NAPA here is usually the most expensive place to buy parts--not always though...I bought new tie rod ends & the adjusting sleeve for my truck from the one nearby for $75,which was less than all the other stores locally,and Rock Auto..they got them in the next day,they did stock the sleeve..
I also got a rebuilt starter drive from them for $35 for my diesel starter,when all the other places only listed new ones for $70,and had to order them..they had it at 1 pm the next day,got it from a warehouse in Maine!..but some of their prices on other parts are a lot higher than the other stores--one plus is the countermen have worked there for decades ,and know their stuff,and their experience can be priceless sometimes..
I also know some of them from my days as a parts man,so they usually give me a good discount..
I often buy used parts from salvage yards too,for items that don't really need to be brand new--just "good"...much cheaper and they have things that are a pain to get or impossible from a store,like leaf spring hanger brackets and other small parts no aftermarket ones are available yet too..the square body supply is very limited here in junkyards now though..