Having recently looked at equipment for purchase, I'm in the dilemna (ebay or not?), from my observations and price comparisons, I've found Regs online seem to go for quite high prices, and some of the regs I've seen are out of service, or soon to be, and 5 years old or more. Add onto the price of the Regs, the cost of service, and anticipated spare parts, and you may find that you are paying only a few dollars/pounds short of what a nice new reg-set will cost, remember it may have been sat around full of salt for a year or two, and the hoses may be perished.
As for BCD's generally they are something that you want to try on, for comfort, you don't know whether it leaks, whether the dumps work on it etc, again, the prices of them online may be cheaper than new, but is it cost effective.
I bought my semi dry suit 2nd hand from my LDS, I got to try it for size, and inspect the state of it.
Rather than go the e-bay route, I would suggest approaching your LDS to check out their ex school kit, many reputable dive schools keep their kit well serviced, and replace it every few years, you can also try it out before you buy it.
Just remember, If you buy something and it has an annoying problem you are either going to have to pay more money to get it fixed, or end up getting rid of it and buying new anyway, and remember, regs are your life support system, do you want to take chances ?