He did not say at any point that he is going to start a large business initially. If he is escrowing 100$ only, then all his trades will be under 100$ else there is no point to open a trade of 500$ with 100$ in escrow.
Also to the OP may be know which centralized platform you will be using for this currency conversion ?
You are right that op has not told that s/he will do a large business but probably you have not seen which they will do? S/he will trade with multiple currency. If you see the currency names (payment gateway) then you would be able to understand that this type of business is not possible to handle with few hundred dollar only.