I'm not sure if it's me, but the going rate for bells to TBT (or vice versa) seems to have gone down a bit. There's maybe 1-2 people offering high prices, but they tend to buy TBT very quickly. (As in, they want 200 TBT urgently, pay a high price, and close up shop.)
A while back, I remember there were big players in the conversion of TBT to Bells. They offered 2.5 mill - 5 mill per 100, but it seems that they've all upped and gone > u < (If I'm incorrect, please tell me.)[/
A few months ago the exchange rate was in my eyes quite sensible, then members started using TBT bells for buying and selling items other than signatures, collectibles etc. As more members wanted TBT bells the rate started to rise and settled for a while around 800k in game bells for 100 TBT. Then Christmas happened and a new wave of ACNL players emerged and joined the TBT bell marketplace in wanting the currency and the rate started to rise again with players outbidding one another in the rush to get TBT bells.
There had been a number of longer standing forum members buying TBT bells on a regular basis, myself included, but as the rate was pushed to extreme levels of several million in game bells for just 100 TBT, most of us if not all dropped out and so I believe you are now left with as you say members who only want a few TBT bells for which they will pay extortionate rates and then they close up shop.
I have my doubts that the exchange rate will ever return to a sensible level unless members start lowering their expectations which my crystal ball says is unlikely to happen.