Bitcoin Forum

Anyone knows a site or [someone] that can sell BTC or any other crypto at or below market prices?

P2P platforms are your best bet. You can also use this forum's Currency Exchange (https://bitcointalk.org/index.php?board=53.0). I highly doubt you will see anyone willing to sell below market price unless they are in a hurry to cash out but best of luck.

P2P would be the only option available to you, however finding a seller at prices below market price is tough and you might have to waste a lot of time getting that order filled even on well known p2p exchange sites.

People sell bitcoin at profit not at loss. Still you can get a deal done depending on how low you want to buy it at.

I suggest you open a thread in the "Currency Exchange" section of this forum and make sure to use an escrow the mediate the trade. Never send money to any forum user who is a newbie and has no prior trade history. If that sees no responses, visit the p2p sites like localbitcoins, hodlhodl etc.

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.

The market price is basically an average of all the buy and sell prices. This means that if you buy at an exchange with really high daily volume you'll basically buy at a price very-very close to "market price" (you can say that you've bought at market price). The only problem is that you may have to also pay various fees (especially at centralized exchanges).
So, as already said P2P platforms may be your best choice.

Of course, you may find sellers on the forum too, but you'll have to be careful (not all are honest) and you'll have to also offer non-reversible payment (since you are new and nobody knows you). In this case you'd probably have to tell clearly the amount you are interested in, method of payment, maybe website of choice to decide the price and... use the Currency Exchange (https://bitcointalk.org/index.php?board=53.0) subforum (as also said).

Just create a buy limit order on one of the exchanges at the "lower" price you expect to by the coin and then hope that the price drops after a while to fill your order. Crypto prices are so volatile so if your order is within a meaningful range then I am sure it can be easily filled.

If you use p2p exchanges, most traders will actually be advertising to sell of their assets are prices higher than the market price. Maybe what you could do is create an advert seeking to buy a coin at a price lower than the market price but be very sure it will hardly attract anyone who wants to sell off the coins at a price below the market price.

Exchanges have a limit when making a buy/sell order, if its the set price is too low, it will not proceed the same when set price is too high.

Better option is p2p, as explained above posts.