i bought LOTS of lots & built a world type set of over 3500 different coins in about a year and a half. it was fun but in the long run it wasn't cheap (more so now with the increase in postal rates) but I learned a lot.
on
ebay the first couple dozen batches yielded many different coins, but after collecting around 125 different countries the "common" lots stopped giving anything new. had to buy a lot more lots to get past 175 countries, and most of those were in smaller groups (20-40 coins at a time). anything past 175 countries you will have to buy one at a time. if I had to guess I probably cycled through 20,000 coins and maybe found 200 worth $10-$50 each, the rest were 25 cents-$1. if you go this route you are not going to make money.
I've since sold off 99% of the collection and am concentrating on one from each country (quality over quantity)
best suggestion if you want to buy on
ebay in bulk is NEVER buy if the seller is using stock/generic pictures. most (if not all) are parking meter rejects or left overs that have been picked clean, and you will get tons of duplicates. these lots go for about $8 per pound (100ish coins) plus shipping.
try to stick to smaller lots, or premium mix (another collectors left overs with a wide range of dates instead of modern common crap) where you can see every coin that you are getting. expect to pay $15+ per pound.
either way have fun and dont expect to make anything flipping the stuff you dont want to keep (in fact expect to loose). if I had to do it again I would start with a few pounds to get my feet wet then head to coin dealers/shows and did through their "scratch bins" (you will get what you want and learn a pile while talking with the people).
good luck & enjoy!