For me, I tend to never steal a user's transfer or overbid a card they want for that reason alone, especially if I'm familiar with them.
It's usually easy to gather the bank on your own or buy the card later, but it's much harder to mend spoiled relationships once you irk them too much.
As for transfers (which, if anyone needs the definition, is pretty much just selling a worthless card for a high price, using your puppets to buy the card(s) from your main), since we're now touching on this topic, allow me to personally recommend some strategies for safe bank moves:
In addition to Mikes Truth's 99-cent increment method (in which you sell low-owner cards for 0.99 bank), here are the two I found the most appealing to me:
1. Gather, at minimum, 2 low-owner cards (preferably ex-nation ones), and sell the first copy for junk. Bid at that junk price, and let the auction run for 59 minutes. At the last minute of said auction, transfer all of your bank with the other copy of the card you had. This leaves only a minute, at most, for someone to underask and steal your transfer (fun fact: underasking/stealing someone's transfer is known as heisting to us card players).
Here's an example of the strategy I'm referring to: https://i.ibb.co/RBDwLVr/Fire-Shot-Capture-171-Nation-States-Trading-Cards-www-nationstates-net.png
2. Alternatively, you can also try out Refuge Isle's method by purchasing a ton of copies of a particular card, and transfer, at most, 1 bank for each copy of the card. If someone underasks one of the copies for 0.99, then buy it immediately, since more copies = more transfers in the long run.
And once you're done transferring with those copies, you can just gift them back to your main and repeat the process (as seen from this example here: page=deck/card=1944609/season=1/trades_history=1?start=25).
But what if you don't have slots to have them all gifted back? In that case, you'll likely have to buy them all back with your main, being forced to counter possible bidders who are interested in the card's market value, or something even more complicated. Best to keep your gift slots open if you consider this strategy!
And yeah, those are the methods I found the most beneficial. There's more out there, but I've seen them be heisted too easily. The ones mentioned here are the safest and most reliable, at least in my opinion!
Now for the other post:
Depends on what you, the player, value personally. For me, I value nature, so I made this collection to keep myself happy.
If you like a particular item (such as a vehicle), then buy as many of the cards that relate to the item as possible. Same thing with players in this region or cards with a specific pattern. There's plenty out there, and I can even direct you to some examples if you need further assistance.
But yeah, the game has a chance at becoming boring unless you decide to start collecting cards that you are passionate about. I'm passionate with both ex-nations and tree cards; what do you enjoy seeing?