MIt depends on your business plan and the method of operation.
Some people do well by buying large collections, cherry picking them and moving the rest at a small loss.
The money from the sale lets them buy more collections.
If, instead of selling today in the hope that a book or two will increase in value down the road, they end up holding all books- two things need to happen.
1) A lot more storage space, which increases expenses
2) They need to replace the income stream they no longer are generating.
If someone has a successful business, its foolish to ditch the whole MO in the hope that a few obscure books might go up in value.
If I hold all the books I get ,hoping some skyrocket, I'll soon run out of operating funds.
This goes back to the argument that Mile High Chuck should have sat on the Church collection.
Comic sellers sell. It's what they do.
Can holding some books work out? Yes, but that's not really important.