why bitcoin transaction confirmation keep coming

[o_e_l_e_o]

i sent a transaction  yesterday and it has about 100 confirmations. Sent from my wallet to an account of at a market, but still isn't showing up in my balance

If it has 100 confirmations then the bitcoin has been sent and received. Double check that you have sent it to the correct address. If the address is correct, then either there is a problem with the user interface or database of the marketplace, in which case you will need to contact their support, or you have been the victim of a scammer or fake website.

[crypto_geek55]

but I have seen that if you visit https://blockstream.info/ after few days, confirmation are still coming.

As long as miners are mining and bitcoin is still functioning, the number of confirmations on all transactions will increase forever.

Can someone why miners are confirming a transaction that is already verified by 5 miners. is it waster of resources and efforts?

They aren't, and it isn't wasting anything.

The process of mining a block requires certain pieces of information. One of those pieces of information comes from the previous block (technically speaking it is the hash of the previous block header) and if anything in the previous block was changed, so would that piece of information, rendering all blocks built upon it invalid. Therefore, when a block is mined, it also serves to verify the previous block (and, therefore, every block before it right back to the first block).

Lets say you make a transaction, and it gets mined and included in block 620,000. It now has one confirmation. The miner who mines the next block (620,001) doesn't include your transaction, doesn't care about your transaction, doesn't process it, might not even know it existed. All it cares about is this small piece of information from the last block. When block 620,001 is mined, it builds upon this piece of information which means that your transaction is now two blocks "deep". If somebody wanted to alter your transaction, they couldn't just alter the block it is in (620,000), because that would change the piece of information that block 620,001 is built on, rendering them both invalid, meaning an attacker would have to alter two blocks - hence two confirmations. Block 620,002 will build upon the information from block 620,001, which built upon the information from block 620,000. And so on. In this way, each block serves to "confirm" the blocks which came before, without wasting any resource or energy on the individual transactions.

extremely hand info. so if I receive any BTC payment, after how many transactions I can say with confidence that its confirmed and cant be reversed.

[o_e_l_e_o]

so if I receive any BTC payment, after how many transactions I can say with confidence that its confirmed and cant be reversed.

The most common number you will see suggested is 6, which will take an hour on average since each block occurs 10 minutes after the last on average. Note that this can vary, as blocks don't occur exactly every 10 minutes, and at the extremes can vary from a few seconds to over an hour after the last block.

6 is an arbitrarily chosen number, though. If you want to be really secure you can wait for more, but realistically, you are safe with fewer than 6. Many exchanges wait for 6, but some will be happy with 3. You can play around with the numbers on this site: https://people.xiph.org/~greg/attack_success.html. If an attacker has 10% of the hashrate (proportion of 0.1), and you wait for 6 confirmations, then they only have a 0.02% chance of reversing a transaction.

Personally, it depends on the amount of bitcoin you are receiving. If I was sending or receiving a couple of bucks, I'd be happy after one or two confirmations, because if the transaction was reversed it's not a big deal. If, on the other hand, I was sending or receiving a few thousands dollars worth, then I'd be waiting for 6 confirmations. It's entirely dependent on what you think is acceptable risk.