That is a very good question. No right or wrong answer of course, but more a matter of your own preference. I also love pins in rl so I think its fun to see what they come up with in the game. The majority of the time I don't buy things as an investment. I usually just buy what I happen to enjoy, and then I buy one or two extra for trading or gifting later. Its so hard to speculate what might have trade value as we never know when they might surprise us and re-release an item or pin. My opinion is that the regular pins aren't quite as much sought after as certain magic and furniture items that you can actually do something with. As mentioned before though there are a few of us pin collectors/traders around so if you enjoy pin trading I would say jump on the band wagon! If you do a little research you will see that vmk actually had a pin trading event twice. I really enjoyed it a lot for the pin trading aspect, other players were more into designing a pin trading room that was judged by staff for a best guest room prize.
For me the bottom line is getting things I enjoy and if I get an extra and happen to be able to trade it for a good value later thats fun too. Hope you're enjoying the game a lot!
BTW, what is your room name? I'd love to come by and say hi in the kingdom