@Islanzadi38 — The default condition for a list of cards is whatever you set the default to when entering them. The default doesn't have to be NM. I'd recommend setting your default to LP or even Played (MP on other sites). If you undershoot condition, you're less likely to have an issue with buyers. And you can still have a statement in your profile explaining this system, so people know cards will often be a better condition than listed.
=====================
@Commander_Bacon — You said it yourself: It's the seller's responsibility. Opening yourself up to potentially greater profits by selling on multiple platforms simultaneously opens you up to the greater risk of selling out of inventory. If you desire the former, you'll unfortunately have to deal with the latter. And a misrepresented inventory that leads to a cancelled sale is, from a buyer's perspective, the very essence of a negative experience, so the negative feedback, while frustrating for you, is 100% accurate. If I need to order cards for an upcoming event, I'm going to avoid a seller who has a track record of having inventory issues and cancelled orders, because I need a reliable seller.
A couple options: 1) List only partial inventories on both platforms. If you have 20 copies of card X, list only 10 on Deckbox and 10 on eBay. This can also help with card spikes; see my response to Sandra below. 2) Give the buyer an option. Send them a message explaining the mixup, and offer these options: They can either wait for you to acquire the missing copies of the card to send them, or they can get a refund. Most buyers will be a lot more reasonable when they feel they have some amount of control in the situation, and you'll be earning their positive feedback by being willing to potentially take a loss to fulfill the order.
=====================
@Sandra — A couple options: 1) A technique I've seen sellers use to hedge against sniping or card buyouts is to only list part of their inventory. So if you actually own 10 of card X, only list 2-4 of it. That way, if a card spikes when you're not looking, you'll only lose out on that extra profit for a couple cards, and can now list 2-4 more of your cards at the higher price. 2) Not all 90,000 of your cards need to have their prices checked and updated diligently. Only a small subset of those cards are likely to be prone to spikes. I would make a list of your rares/mythics, older foils, vintage cards, cards that have only had 1-2 printings a while ago, and the like, then keep that list up-to-date on a frequent basis.
Some other comments: In one of your posts, you stated that you "can't take a hit like that", referring to a $1 card becoming $20. But.... you're not taking a hit. Unless you bought the card for $5, then were forced to list it at $1 for a loss because the card went down in value, but now missed out on a spike that would have turned that loss into a gain.... unless that's the scenario, you haven't taken a hit. Presumably, you've still made money off of that card relative to your initial cost basis. @ic0n67 hit the nail on the head: If you were happy to sell the card for $1 prior to the spike, then for you, it's still a positive sale.
You also stated that this is your only source of income, which implies that you view it as a necessary means to bring in money, aka a job. Thus, it would behoove you to treat it like a job. Since you don't have any other job competing for your time, that gives you more freedom to spend the hours you would have spent at a job on this instead. Now, I'm not assuming you can spend 8 hours a day on it! Perhaps you have multiple kids, a house/garden/etc to take care of, an invalid relative you care for — I understand, as I have 5 kids and an 8 month old, we just started homeschooling all of them due to the pandemic, and at the same time as all that, I'm trying to work from home. What really flabbergasts me, though, is that, in your profile, you state that "updating the sell list daily or weekly simply isn't something I have the time to do" (emphasis mine). I get daily, but if you don't have the time to do it weekly.... then frankly, you have no room to complain about card spikes. The amount you earn at a job is directly proportional to the amount of time and effort you put in. The same is true as a peddler of cardboard. If I were to start selling my collection right now to make extra money, I know I wouldn't be able to treat it like a full-time job, so I'd resign myself to the fact that, if I missed out on some announcement or top 8 that made a card spike, oh well. I missed out. If I can't spend the time to ensure I run my cardboard-selling business correctly, then I deserve to experience losses.
=====================
General comment to everyone asking for "sale proposals" — As @asrttyoxo said above, no other marketplace does this. And for good reason. How long is the buyer supposed to wait on you? Hours? A couple days? How long before they get the chance to cancel their order and ding you with negative feedback on that failed transaction due to nonresponse? And what if they wait 20 hours, only to be told, "Sorry, we don't have enough of that card in stock" or "Sorry, I had the wrong price listed"? If the card IS in the middle of a buyout, the buyer may have missed other opportunities with other sellers while waiting on you. Or if they were ordering for an upcoming game night, but then have to wait two days only to get their order proposal declined, they're gonna be rightfully annoyed with you. Yet in your proposed system, they'll have no means of leaving feedback that warns other buyers away from a slow seller. Honestly, I'd never again buy from anyone on Deckbox if this system were implemented, because I value the timeliness and dependability of the transactions.
It sounds like everyone just wants an extension of the trade functionality that allows trading money for cards instead of cards for cards. In this system, I could look through your tradelist, add some cards to a trade, see both the Deckbox-calculated price as well as your listed price you're asking, and make an offer along those lines, much like eBay's Best Offer feature. Trades could also be a mix of money and cards from my tradelist, if you've stated in your profile that you'll accept trades that aren't cash-only. Once I propose a trade of my money for your cards, you'll have the option to accept, reject, or counter my "best offer", just as you would with card-for-card trades.
Basically (and @BluePrecision), if it's a marketplace with sellers, then it needs to function like any other store; it's not a different business model from TCGPlayer or MKM. If I click a button to buy, then by the very fact that you have the card listed for sale, you've agreed to sell it to me at the listed quantity and price. It is 100% the seller's job to ensure that can happen.