Can I lock a transaction from being used until a particular condition is met

Question

I plan to open an ecommerce website from which I will accept BTC as payment. However users dont want to pay with BTC without getting the goods since my site is just starting up. What are the options I have?

Answer 1

Your question indicates some experience with bitcoin, so I am going to be very technical. You may want to look up "smart contracts" as keywords. In the bitcoin world we talk about checklocktimeverify and checksequenceverify, together with some multisig (as escrow service). There are many examples, e.g. here (this is basically what I posted in bitcointalk):

IF
    2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIG
ELSE
    "30d" CHECKSEQUENCEVERIFY DROP
    <Alice's pubkey> CHECKSIG
ENDIF

or something like this:

 if
  <merchant pubkey> checksigverify
 else
  <timestamp> checklocktimeverify drop
 endif
 <customer pubkey> checksig

imho, n-of-m multisig is the easiest form of a contract. Escrow service is the next layer (as in examples above). They can make sure, that BTC is only payed, when a specific condition is met.

And then one can create very specific constructs like:

CHECKSEQUENCEVERIFY: lock some Satoshis until a specific number of block have passed, before one can use them

CHECKLOCKTIMEVERIFY: lock some Satoshis until a specific number of block appears (aka my son is 18 years old...), but use a 2-3 multisig to release the funds in case funds are required earlier

It goes even up to the point where you can create tx spends, where the hash of a condition must be equal to a previously defined value. I see some limitations when trying to use Oracles (aka data from "outsides"), maybe data from exchanges, or from real live (in case of death).

Some more example in Andreas' book "Mastering Bitcoin" (second edition) in the chapter "Timelocks".

or here:

https://bitcointalk.org/index.php?topic=1300723.0

https://bitcointalk.org/index.php?topic=1558207.0

https://bitcointalk.org/index.php?topic=1952248.0

or here:

https://www.reddit.com/r/Bitcoin/comments/4kit49/is_it_safe_to_use_cltv_for_recurring_payments/

https://www.reddit.com/r/Bitcoin/comments/4p4klg/bitcoin_core_project_the_csv_soft_fork_has/d4i01he/

and here:

https://en.bitcoin.it/wiki/Timelock

https://en.bitcoin.it/wiki/Contracts

Answer 2

Possible solution(s)

Depending on the kind of goods you sell, the size of your company and the prize of the goods you sell you could look into Ethereum and smart contracts.

If the goods are not digital you can't use smart contracts unless you add Digital Twins to them. Digital Twins will work for cars, maybe branded clothing and similar goods, but not for oranges or peanuts.

Hope I could help you a little bit, but it's difficult with so little information about your site.