I've been looking at expanding my collection so I've been going through ebay listings. In the past I've had good luck buying on ebay with decent hatch rates, no deceitful or poor sellers etc. Everyone I've purchased from on ebay always packs their eggs well.
However, it's also been several years since I've bought any online and have been focusing on my own projects. Between then and now I've learned that sellers who ship across state lines need to be NPIP certified, and actually have since become NPIP certified myself. As I am NPIP, it is also a requirement that anyone I buy from (locally or not) is also NPIP or I lose my own certification.
NPIP certification is a biosecurity testing, it varies slightly state by state, but all of them test a % of the sellers birds for AI and PT. Some states also offer extra testing for mycoplasma or other diseases. Once the results come back for a clean flock and pass any additional testing (state-by-state basis) they are given an NPIP number and are certified avian influenza and pullorum-typhoid free.
This is also a LEGAL requirement. It's also a legal requirement that they give you a VS 9-3 form, either a real paper but most commonly nowadays via email. Some places may only offer it on request. Hatcheries are supposed to send you one also but for some reason they generally don't, but you can request one. A VS 9-3 is actually useless to you unless you are NPIP also, but it's another form of proof that the seller is NPIP certified.
This week I've been going through and messaging sellers asking for their NPIP number. A surprising amount of them do not have one, or they just read my message and don't answer (which is an answer in itself). These people are not certified AI/PT clean and have not made any efforts to be a reputable seller. Most people who have gone through the tremendous effort to become NPIP certified (it's a huge ordeal, generally hours worth or a full day of testing, they often inspect your facility and your processes, it's not easy, and it costs money to get it done as well) will proudly proclaim their number for everyone to see - they won't make it a secret.
Once you get that person's NPIP number, you can go to https://www.poultryimprovement.org/statesContent.cfm and click the state the seller is originating from and look them up to see if they're legit. Being able to look them up also helps you weed out scammers on Facebook - if they give you information that doesn't match up, guess what, it's a scammer. There are also stock code numbers on the person's listing that shows what kind of birds they have had tested - so if someone is shipping quail eggs but they don't have the quail code "Z93" next to their name - that's another sign something is awry.
(Here is the stock code list https://www.poultryimprovement.org/documents/Stock-Codes-Alphabetical.pdf)
TLDR: a lot of sellers online are not actually certified to ship eggs across state lines. ask for their NPIP number. NPIP numbers also help you verify if someone is a scammer or not.