i doubt any one is willing to lend more than 25-30%of BTC value, if you borrow 25kfor 1 btc and the BTC plunges like it did in the past,thenthe lender is screwed!
Even at 25-30% it seems to be a bit of a risk for the custodian if the loan is in fiat. Given that we have seen BTC lose 80%+ of its value from peak in past. But that's where the numismatic value of the collectibles comes into play too. If the loan is in BTC value it would probably have to be close to 80-90 percent or more.
for me, I would never do this - this would mean giving someone possession of my collectibles. I see this as the same sort of fallacy as trusting an exchange to be your wallet. It's all good till they have 10 million in physicals that they loaned out 5 million for and then they up and disappear. And that could easily happen in the crypto space. And it has happened many times. Sorry, once you give up your keys, it ceases to be your crypto. You are essentially selling your item and hoping to eventually buy it back once you pay off the loan.
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I would also worry about other potential threats such as a potential fire or theft. Or some employee in the lending firm going rogue and redeeming your private keys (this might not be very likely to happen but you still need to put some level of trust on whoever you're giving possession of your collectibles to). When I keep collectibles under my own watch, I understand all the risks and can take measures to protect myself against those risks. I wouldn't trust a firm or custodian whose sole purpose is to make money to make decisions that are in my best interest.
I personally would never take a loan against my collectibles, but different people have different risk taking tendencies so there might still be market for this. People trust the likes of Celcius and BlockFi to lend their BTC and borrow against their BTC, I imagine there would be a market for what OP is proposing too.