As for operating with a single account to stay within the vendor’s technically-enforced limits, I believe this is the current state of things.
Cliff notes: you’ll be fine as long as you don’t go on a spending spree at Target.com, and try not to bring attention to your buying patterns by dealing with online customer service for small gripes that can be dealt with in store instead.
Target - the easiest place to get banned based simply on purchase volume online and the most aggressive against resellers in store — employees trained to identify resellers loading up carts for example. Can still use red card in store after online ban, but using with a new email account will result in repeat ban — start receiving auto cancellations and email about possible issues including payment purchase method. Some people have bans lifted after a period in timeout. Others indefinitely. The amount of purchases that may identify you as a reseller and get a ban? One data point: I have been unable to purchase online going on 18 months now after two months of $5k purchases on the red card. Same red card still works fine in store.
LEGO shop at home — with the purchase limits mechanics implemented a few years ago, it is much more difficult to get the SAH ban hammer. If you exceed a limit on any item based on past purchases of the same item, they will cancel your entire order. Be warned that if they cancel an order with a VIP Rewards purchase, the reward is not automatically credited back to your account. Also, if they cancel an item due to inventory issues, the system still tracks it as qty purchased. Qty purchased in brick and mortar stores that are purchased with VIP accounts are not counted towards SAH limits and and limits in store are done at the individual store’s discretion. If you do have to call customer service to resolve an issue, they do have easy visibility to all of your previous orders and will make comments about you “really liking LEGOs”. No indication that that is how the ban process starts, but unless you really have an issue that needs CS, might be best to just let the small things go.
Amazon — shop away. Their limits are tracked to the second on a 168-hour FIFO. Limits apply and are tracked in that window only to items sold and shipped by Amazon.com. Buy as many third party items as you wish. Anecdotal stories of bans for excessive returns, but no clear information on the metrics used. Generally, probably best to save the returns for the really bad boxes. Creating multiple accounts will get noticed and is the quickest way to get the warning shot emails.
Barnes and Noble — similar to LEGO, has a five qty per item lifetime limit online. Exceed that amount and just that item will be cancelled from the order. No reports of bans. In store purchases do not appear to be tied to online memberships as far as adding to your 5-count quantity.
Walmart — most LEGO are limited to two per order, but they won’t say how often that limit is reset. Doing a couple of multiple limit orders in the same day has worked. Doing dozens has resulted in order cancellations of those specific items but no bans that I am aware of. Generally, I have found one limit order per day has never resulted in a cancellation.
ShopDisney — some items have limits but they are selectively enforced. If you put in multiple limit orders at the same time, they might cancel those items. If you space them out, they will be fine. I ordered a dozen castles (limit one per customer) last year and they all went through. Sometimes two in two consecutive days.
Costco — has item limits on some items, but I haven’t seen any on LEGOs. Even with items with limits, possible to space them out without issue. In store limits are usually “per day”.
Best Buy — unsure on LEGO as I’ve never pushed the limits on them. In the past, I would make 50 purchases at a time of 2-dollar items that had limits and they would all go through. I have tried to make multiple purchases of Xbox consoles and those were cancelled. So YMMV. But no reports of bans.
Anyways, those are my observations.