I hobble down the main street in my neighborhood, going from one bookstore to another (there are only two). One is a used bookstore which stocks plenty of bargain to fill in the gaps. The other is a new bookstore. Back when I was a sales rep I tried a few times to sell to the new bookstore, but the owners had no interest.
Despite my constant cajoling, I do get it. You have built your reputation on all new books, frontlist and backlist, author events, the ability to return most inventory for credit, preorders, and everything sparkling clean and brand new. Bargain can be daunting. Here are another 30,000 titles to sift through, looking for what will work in your well-curated store. And you must have the courage of your convictions -- no returning for credit. So keep doing what you're doing and make it the best book experience you can for your customers and staff. I still think you're crazy, but good on you.
You knew there would be more cajoling, so here it is. I was looking for Silent Cavalry by Howell Raines. The used bookstore didn't have it, even the public library didn't have it, at least that day. The new bookstore also did not have it. They offered to order for me, but I decided to wait.
This was a weekday morning, maybe 11:00. The used bookstore had about 15 people shopping and two buying books. When I went to shop in the new bookstore, I was the only customer there.
If you had one bargain table with short stacks of between 30 and 50 titles, and then 2 each of another 50 titles to shelve in your sections, you will have invested enough to give bargain a fair shot and you will not break the bank to do it.
And don't worry about the time it takes to parse those lists and websites. Some of the best sales reps in the business are working for bargain wholesalers. At least for the first order, let them put it together for you. They are motivated to make sure your first order is full of their best sellers to indies.
Price everything 50% off list price to start. If you're paying $4.00 for a $25.00 book and selling it at $12.50, your margin is good, and you can afford future markdowns if necessary. I would advise marking these with stickers and entering them in your inventory system, "R" before the ISBN, but if that's too much, just dot them and make sure your customers and staff know that these are 50% off list price.
This is just a start, getting your toes wet before jumping in the pool, but you will be pleased enough with the results to come back for more.