Anyone knows a site or [someone] that can sell BTC or any other crypto at or below market prices?
Why you are interested in the bitcoin price? The price is the most volatile thing in Bitcoin and the amount that may be lower than the current market price may be higher after a few days.
If you intend to do a trade where you are trying to profit through the price difference between the platforms, you need some proof of identity and may make losses because the bitcoin price is highly volatile.
In short, unless you have a logical reason for this, trying to buy at a price different from the market will not be profitable (financially or in terms of time)
people buy/sell bitcoin less than market price when they ant to hide them identity.