TCC uses a mean of all the global listings. MM is limited to the guild(s) you are currently in.
But I use TTC's website to find things to buy, and the addon to list my guild's stuff on there. If you mean selling stuff through guild stores then IMO Master Merchant is best for working out prices, because it uses data from actual sales whereas Tamriel Trade Center goes off items currently listed - which may be hugely overpriced and highly unlikely to ever sell. You should probably set a starting price slightly below what you think it's worth to get the bidding going, but then just wait and see what happens. If you're selling things by auction then you don't need to worry about getting the price exactly right, because it will go for whatever people offer to pay for it.
