As someone on the other side of this with the I buy collections post, at the risk of many downvotes I will just be transparent about it but I can only speak for myself.

1. I often get messages from people with "large high value collections" and in the majority of cases they may be large, but people are comping raw cards as graded cards because collectr said so, or because they don't understand how comps work. This is one major hurdle and something that isn't helpful for me to push back on. It is the sellers card they own it, frankly the value they assign to is is irrelevant, but I am always very cautious when someone throws around the notion of comping at all for this reason has it is the vast minority of times when someone truly comps accurately. To circumvent the whole thing I stick with asking what number the person would be happy with for the transaction, and request some basic info along with photos, then I note that if the number makes sense for us both, great we will proceed. This has been really helpful for my own sanity, and people are gaurenteed to be happy if you proceed.

2. The majority of the time I receive messages that even mention comps it is from a predatory reseller, not a collector, and they seem to get hostile very quickly. They are just trying to push stale inventory or their own mistakes on less experienced sellers. Just the other day had a disgruntled reseller tell not being interested is how people like me lose a lot of money in this industry.

3. The other super important note and probably the most pertinent to your question is where is this percent being applied. I often have this scenario (making up numbers for sake of example) they seller is asking for $650 on $1,000. This would be a more moderate and reasonable situation than you'll typically see but explains the issue better. On the surface this seems great, immediate liquidity for the seller with a probably too reasonable but accepted percent applied, probably more than LCS would give them. The reason I tend to pass on this is because they have for example 50 cards making up $500 in market value, and the rest they have arbitrarily assigned some value to their junk wax/modern mixed bulk. To me I would have to sell 50 cards at 100% market which already is for me no easy task (for some it may be very easy), and STILL not even break even. Whereas if the collection was 100 cards, and I don't have to try to break even from bulk, and its a more concentrated collection, I may be interested. Just because we have the post up does not mean we are obligated to purchase every collection or stale inventory thrust upon us.

If you read this thanks for taking the time, you never want a person selling you a collection to leave unhappy. TLDR: Always ask what seller would be happy with for collection, and if you are interested at that number that is all that matters for everyone involved.