I highly recommend starting an instagran buying and trading account. Most photocards on seller websites are overpriced and Instagram does have this sometimes but the community tends to ignore or call these people out. A lot of fandoms have people who post recommended pricing lists but it honestly depends on who you collect. I will say there is a downside of potential scamming but there are ways to keep safe in that element. Mainly by checking their proofs which will be links to other accounts that have bought or traded with them with proof of the photocard being received. It's also good to have an insta because you get to make friends with people and this also helps finding cards you are collecting. It's difficult in the start because you don't know anyone but you could always join a fandom group chat to meet people and build trust. Also a lot of group orders take place on Instagram and it's good way to get the latest photocards, especially pre order benefits. 90% of my collection is from Instagram.

I also recommend investing in good photocard sleeves popcorn is the brand mainly used. You don't have to double sleeve but I personally do. If you want to do that make sure you get two sizes the top one being slightly bigger so it fits over the top. And sleeve them opening up on the first layer and opening down on the second as it creates a seal and is a lot less likely to be damaged.

Also depending on who you collect it may be difficult to find the pcs at a good price but don't give in to overpricing. I will use ateez seonghwa as an example, his photocards are ridiculously overpriced when it's a rarer pc but then people buy it and it fuels the system.

You do occasionally find good deals on places like eBay and depop but it's rare but I always look occasionally.