The easy answer is, whatever the market bears. The real answer is that it depends on how you choose to sell them. There are about four or five ways to value a collection.
Let’s say Bob has a collection of 4000 LPs, and he specializes in ‘70s soul and funk, but also has some other related genres. He’s made an effort to buy minty records, upgrade the lesser copies, keep his records in plastic sleeves, and has a decent turntable. In other words, he has a quality collection. Bob is a realistic guy and thinks he knows what records are worth. In his mind, his collection prices out at around $100,000. He’s figured this out because he did a random price sampling on eBay and Popsike of 200 of his albums and the average price per record was $25. He then multiplied this times his 4000 and voila, $100,000. And you know what? On the surface he’s right.
If Bob sold his entire collection piecemeal on eBay he would net that amount. However, it would take him at least twenty hours per week over a year’s time to photograph, list, process, pack and mail his records out. Then he’d have to pay eBay and Paypal fees (10%?), and pay income tax on his earnings (20%?). So his target number is shrinking. Plus, you have to figure that Bob’s time is easily worth $10 per hour, so subtract another $10,000. Now he’s really made $60,000. It doesn’t mean that his collection isn’t worth $100,000, it’s just that even taking the best monetizing trail, you end up with a 3/5 payout. He might also have to factor in a therapist, because what condition is Bob going to be in after spending a year selling his prize collection?