If I were to connect a 3rd party e-commerce solution (Shopify / BigCommerce) would they get live inventory to display on my retail website?

Yes Celigo has a Shopify connector that syncs your inventory from NetSuite into Shopify

Are the changes synced real-time? Or on a schedule?

It would be on a schedule up to every 5 min. The reason being there isn't an event trigger available (that I know of) in NetSuite for when quantity availability changes.

So what happens if the stock levels are low, and we take orders over the phone? It would over allocate.

I guess if you are having frequent checkouts online and taking orders over the phone at the same time there is the possibility of getting an online order with a back order. But it won't over allocate

That is a possibility for it to backorder upon import, but if you are syncing every 5min then it would be pretty minimal. The alternative is to factor in some safety stock when publishing inventory to the website. For example, website inventory may be total available inventory from NetSuite minus 2.

We do retail and b2b so someone can wipe out a lot of inventory very quickly, that's why I'm nervous about the stock not being up to date? If I understand correctly we would be storing stock levels in 2 places? 1 in netsuite, and one in shopify?

Yes, but NetSuite would be your source of truth for stock levels and Shopify would be delayed by 5 min. Overselling is going to happen from time to time, but can be mitigated by syncing faster and/or adding safety stock to the website.

Yes I agree every 5 min should be sufficient but I don’t know three frequency of your orders I think a real time refresh after Save of every transaction would significantly degrade performance in NetSuite. So that wouldn't be great

If I were to use the netsuite api: https://stackoverflow.com/questions/61635204/how-can-i-get-inventory-item-quantity-from-all-locations-by-calling-netsuite-res Would shopify integrate with this? This would pull the data as opposed to pushing it, so we shouldn't have a performance problem with netsuite, although the website might be slower.

That could theoretically work, but I’m not sure if Shopify is capable if doing that. You may have to build a shopify app

Headless?