1. people who have lots of tbt have most likely been around on the forums for quite a while. either that or they may own a shop- many shop owners sell unorderables and gardening items, which are in really high demand, and so they make a lot of tbt there. there are also people who sell art, so members who make signatures or icons are also in pretty high demand and make a good amount of tbt from each piece of art. i'm not sure what you mean by "where did the bells come from originally," but people also make bells just by posting, so i guess tbt originate from there
2. for me, i like using tbt because it is easy to transfer between players (just a click their bell amount and enter the amount you want to give type of transaction), while using igb takes a lot more time (the paying player has to either set a ton of bell bags out on the ground or put a lot of random stuff up for sale in retail for the retail method, and the receiving player has to pick up all the bell bags and make multiple trips to and from their locker for big transactions). it's just easier for me, and i don't feel like i'm getting shortchanged when i get paid in tbt because i know that the general exchange rate is 1 tbt = 200k igb. i always know that i can always exchange my tbt for igb if i ever run low. i don't personally use the shop on the forum, but people like buying collectibles to display under their avatars (like cakes! and fruit! how cute) and also like to expand their mail inventory or be able to further customize their personas. i think i don't really differentiate between tbt and igb, but in the end, i know they all mean the same thing- like if i had 1 dollar in one hand and 100 pennies (i'm thinking american money haha) in another, i would just think "cool i have two dollars" because despite the specific values of each, in the end they all mean the same thing...
yikes i hope that made sense- it's late where i live and i just rambled on and on...