This is how I've operated over the past seven years since I got into collecting specifically vintage cards and I've had some modest success (as in my collection has nearly been completely funded by having flipped cards over this time period). I work in education so I can definitely relate to having limited discretionary funds

1) Study the market of the players you're interested in and know the values of those cards in the range you are thinking of buying like the back of your hand. That way when you see what appears to be a good deal you can act quickly. Key cards of blue chip HOF'ers are a good starting point

2) Only, or almost only, buy cards that you'd be happy owning your entire life and passing on to family members. This will satisfy the collector in you and separates you from just being a business. I've had more great cards go through my hands than I currently own, but the one's in my personal collection will hopefully be passed on to my son one day

3) Buy multiples of cards that you believe are a good value when possible for investment purposes and for your own personal collection. Nothing worse than owning one copy of a card that's exploded, especially if its a player you collect (in my case 1947 Bond Bread Jackie Robinson)

4) Know when to take a loss. I overpaid and owned a really nice M116 Wagner Blue background in SGC 60 for over a year before I listed it on BST and it was clear to me that I had overpaid by at least $1200-$1500. Eventually I took the loss but rolled that money into an E121 Ruth that I was able to sell within weeks and make all that money back, plus an extra $2k

5) Contact sellers when a price is solid and ask if they are able to do any better. This will piss off about the same percentage of sellers who end up knocking off an additional 10% from my experience.

6) Sell when you want to not when you need to. Obviously you don't want to be in a situation that you need to sell cards to remain afloat