BTC double spend

[PatrykCOIN]

I am merchant hoping to accept BTC as means of payment, I need someone to throw more light on the term  "DOUBLE SPEND"  & how it affects my business

[Yakamoto]

I am merchant hoping to accept BTC as means of payment, I need someone to throw more light on the term  "DOUBLE SPEND"  & how it affects my business

The easy explanation for this goes as follows; someone sends a transaction and then "refreshes" their wallet so that it doesn't register the amount as being spent, and they send the same transaction to another address. The best way to counter this is to require 3-4 confirmations on a transaction before accepting the transaction as completed, as this makes sure that they do not have the ability to double spend and get away with it.

It shouldn't affect your business so long as you make sure the confirmations go through properly.

[Herbert2020]

the short answer is: never accept any transaction which has 0 confirmation
the easy method is: use a service like bitpay.com to accept bitcoin payments

the long answer.
technically you don't own the bitcoins you receive unless they are included in a block or in other words mined or commonly known as confirmed transactions. a transaction with 0 fee can be double spent. it is not as easy as it may sound like but it is possible.

as for number of confirmation, in bitcoin with the huge hashrate and difficulty, and when network is fine, 1 confirmation for small transactions but 3 confirmation is more safe for most cases. for large amounts of bitcoin 6+ is recommended and in cases of emergencies 15-30 confirmation is recommended.
read more:
https://bitcoin.org/en/you-need-to-know#instant
https://en.bitcoin.it/wiki/Confirmation
some more technical information here: https://bitcoin.stackexchange.com/questions/1170/why-is-6-the-number-of-confirms-that-is-considered-secure

also there is this API which many gambling sites use when they accept 0 confirmation transactions: https://www.blockcypher.com/dev/bitcoin/#confidence-factor