It's more that the trading post is the same as the hidden tower, really. People don't want to make a loss on items they've bought and chosen to sell on, so they're not going to price them cheaper than what they paid for them from the hidden tower. Trading post prices may be more or the same, or sometimes less, than the prices in the hidden tower - these fluctuate. Prices in the hidden tower remain the same no matter what. Only TNT can change those prices, and they tend to stay the same.

So in answer to your question, yes hidden tower prices may sometimes be lower than the trading post prices - but only if a user wants to make a profit on an item they bought from the hidden tower, and hence priced their item higher.