Well, actually the people doing trades on OTC platforms are not market manipulators. They simply do not want to deal with centralized exchanges, because centralized exchanges panic when they need to deal with large buy & sell orders. The first reaction from most exchanges, when large buy/sell orders are placed, is to stall or to temporarily suspend those people's accounts for some obscure reason.
The buy orders will not have to be coordinated, they can simply arrange this in their own time and we would see a small spike in the price.
A few weeks back we saw a coordinated buy order in several exchanges for an amount ranging to hundred million dollars and i saw that as a spark for the upward movement, if the spark of seeing a huge buy order in exchanges could bring some change to the market, so be it
. I came across a few OTC exchanges where they sell only bulk orders, initially you have to agree upon a fixed price and make the payment and the coins will be sent to your wallet after your payment is verified, minimum buy order will be hundred thousand dollars. I am not sure centralized exchanges will suspend if you deposit a huge amount to trade, never heard such complaints till now.
Basically, Over The Counter (OTC) Platforms are decentralized trading/exchange platform where the sharks will be able to exchange easily and buy a large number of Cryptocurrencies with convenience and less transparency.
These are centralized platform that deals only in huge amounts, you send the money in the account provided by them and they will be sending you the coins in your wallet you provided, basically it is a one on one trade.