definitely have a game plan before you dive in. do the research so you know you are spending money the most efficient way. be patient...this will be a marathon if you are doing it bit by bit instead of all in at once.
1- know what type of conditions you are shooting for? do you just want a set of loose figures in any condition or do you want the best shape possible? do you care about all weapons or some weapons?
this will all determine what lots of figures you are eyeballing to buy. i think everyone agrees, you will get the best deals buying lots so knowing what conditions help you pass on lots in poorer conditions.
2-have a re-selling game plan. if you a buying lots, you will certainly have doubles the more lots you buy. buying lots gets tougher to stomach the closer you get to your goals. it's fairly easy to own the first 77 figure give or take a couple conditions or upgrades but if you have a great re-sell plan, getting the last 17 in a lot is much more cost effective than purchasing each single.
being able to flip all the extras will give you more confidence to make the purchase.
3- know value and know math. when you look at a lot of figures and the buy it now price or bidding price as an auction ends, you have to count up all the items in the lot and divide the total asking price to see what each figure would be. you'd be surprised. it helps you know if you are paying $3 or $10 each for a bunch figures and if you can move the ones you don't need at that price. don't forget about all the other "junk" that in the lot. cardbacks, parts from vehicles, weapons that don't belong to a figure, etc. that has re-sell value also and can help you sway the amount paid into a profit.
4-know when to walk away. you walk away when there is no re-sell value in the extra items.
5-know when to spend. sometimes you will pay $500 for a lot of 20 figures but the lot will have blue snaggletooth and 19 randoms. blue snaggleooth could be a $400 figure down the road so spending $500 to save $400 might be your best option because the other 19 can possible take $150 off your total if you re-sell correctly
6-ebay isn't always the search option. facebook marketplace sometimes always presents a good lot and sometimes into right around the corner. you gotta keep checking and know when to make a move because others are also searching.
7-be aware of whats in the lot you are looking to buy. some ebay auctions run into bidding wars and the lot goes much higher than what you were willing to pay. the reason is, the lot might have something in it you were unaware of that someone else was willing to pay plenty for. it helps to be educated because the person who outbid you is probably a re-sell expert.
8-the auctions that are unorganized usually offer the best values. look for these type of auctions...poor titles, single photos of a pile of figures with weapons. some buyers don't want to be bothered with these but if you have a good eye, it can work great for you.