Sorry i been away and not had time to reply,
An AlternatePrice would be perfect, i have noticed, all merchants sell all items, for Coins (Price) also noticed all merchants that use BP, GBP (Guild Bounty Points - not Great British Pounds
) and Glass (Toa Quest) have the same exchange rate, but ones you haae the items, the sell rate is diffrerent and dont match the first rates, same with other merchants.
You could use a currency converter, but you need one for every type of merchant for buying and a different one for selling.
With the AlternatePrice, you would leave all merchant to sell on the normal Price, but you just can buy in different currencys depending on the merchant.