Captain Stamp - As others have said, we have all made mistakes in our collecting journey. I most certainly have (some painful memories persist!). That being said let me offer you a few words of advise.
1) Education, education, education. Learn all that you can about your subject matter before spending any real money on your material. These days there is so much great info online that was not available when I started. You have an advantage here. And the cost of most books or a membership to an organization like the APS where you can access their library will be a pittance compared to the money it saves you. Vacuum up knowledge. There are so many people right here on this forum that will help you BEFORE you spend money.
2) Think of catalog values as rough guides and fully understand that those values are almost uniformly for VF stamps without any problems. Know that any faults or less than very fine centering drastically reduces the value. This point will become clearer as you do your research as noted above.
3) Tying into catalog values is the "shiny object" principal in collecting, especially stamps. Do not be distracted by sellers dangling large catalog values. Base your offer on what YOU think it is worth to you after researching it. What have similar stamps actually sold for on online marketplaces. The research is free.
ebay sold results cost nothing to view. Use these free resources.
4) You can build a relationship with one or two sellers, maybe even a friendship, but always keep in mind that when money is involved put your interests first. Close families dissolve over money. Assume that someone whose main tie to you involves taking your cash might be trying to maximize what they extract. Sounds cynical but I always reevaluate whether a buy-sell relationship is good for me. This might seem or be cynical to some but unless they are paying your bills who cares.
IMO the most important points are education and not letting catalog values influence you. Put in the work and establish good purchasing hygiene and you will be a happier collector. Too many come here after spending money and become disillusioned because they overpaid or were cheated and they swear off the hobby. They lose and we lose.
Hang in there and make it about the journey.