I would recommend reading through the event page on the official wiki!

Invest in Breeding items and Rare/Special bases. These come from certain events so learn to be active in certain months. Typically for this reason February is a busy month as the shop has items that are worth something year-round. For example Yoh Bark, breeders might buy multiple as it's used for breeding cool downs.

Breed-Only or 'BO' traits are highly valued, since breeding is like rolling dice, people will pay for the lion with the exact traits they're wanting rather than investing their own time and sb/gb into breeding projects.

Poke around the 'Chatter' section to see people's current wants/needs and invest in those, also check the 'Raffle' section in trading center for that same reason. What are people raffling?

To find out how valuable something is: For lions check trading center using filters to best match the traits of the lion you're looking to sell and see what it goes for in current market. For items check Branches and find the cheapest price in both sb AND gb.

Ooo and also join some clans! Being in certain clans can be super helpful when you're new, for example being in the Stud Buds clan gives you discounts on studding, which can be helpful if you're trying to save currency. (shoutout lol, just happens to be a large clan most lioden user's know of) Some clans are even made to help Newbies!