I'm planning to buy a new Samsung phone from Shopee/Lazada as they're way cheaper than official Samsung itself.

I have 3 questions:

1) Should I be worried about seller unsealing the phone box, allegedly for QC and warranty activation?

2) It seems that they will register warranty before shipping, but some buyer reviews said that their warranty date started way earlier (months earlier) than their purchase date. Seller said to take the invoice to Samsung Service Centre to update the warranty date, based on the invoice date. True?

3) Are there sellers that ships it sealed? (except for Samsung Official Store lol)

yes some store seal phone. i bought multiple times from different stores. so far i only bought xiaomi phones at lazada and shopee and so far comes with seal.
key point when i am buying are:
1. reputable stores. good ratings or always can check seller sell many phones and good reviews. example if there is no/less review phone that i want to buy, i will check other phones from seller that being review by other buyers.
2. malaysia warranty and authentic original. if do not mentioned malaysia warranty i will skip.
3. price. if too good to be true, will skip. if cheap should still be in reasonable price and not super far from retail.
4. transaction only via the platform.
5. i will check spec of phones at gsm arena and utube video before decided to buy which model.
good luck TS.

From my research while buying previously, it seems almost all online sellers will unseal it, claiming it's for warranty or QC.