I will answer in parts.
I think skill reset potion is not available anymore (only EA packs, and Begginer’s one had, I think). I didn’t check TP trade NPC for some time, but I believe that you can’t buy. And from what I remember it is not tradable.
Like @Gringe pointed out. I mixed up stat reset and skill reset.
But I believe that it is untradable.
No, TP costumes are only for the character you buy it in (chara bound). You can’t pass to other characters on your team yet, if I’m not mistaken, soon we’ll see this feature (you’ll be able to pass via team storage).
No, buying and selling are available even if you are not a token user. The only difference is that the market fee when selling is bigger (30%, normal user; 10%, token user).
I think that is something for you to decide. Some will say “TP is useless!”; while others, “TP is the best thing you could spend your money with!”. It really is something that differs from player to player.
But with TP you can buy some costumes, pets (in the future, maybe, according with kToS), exp tomes, tokens (that can be obtained via market), etc.
So, you will be able to get some nice things with TP, but I wouldn’t say that you would lose something not buying it.