It can take awhile to figure out the in-game economy; costs fluctuate depending on what units and items are in demand. Depending on the type of event, sometimes everyone will want to buy potions, sometimes time elixers. Spend some time checking out the forum or other players' walls in the game and you can figure out what current values are.
non (P) items can be purchased directly with Fanta Points, and sometimes Event rewards will be non (P).
Gloria is a nice card to set your sights on; I'm trying to find one as well, but I think I need to save up some more Time Elixers first.
I recommend saving every single potion and TE you get until you've got enough saved up for a decent trade. I've seen level 1 4* will go cheap for 1 or 2 TE whereas maxed 4* go for 15-25 TE depending on the tier of the card and/or if it's part of a limited edition series.
When you use potions and TE in the game, it will use (P) first, so if you do spend TE or potions for an event, watch your inventory to make sure you're only using the non-tradable (P) type. I'm impatient so I burned a lot of tradable potions and time elixers early on because I didn't recognize their value.