You should sell; I collect books my self, rarely do they go up in value and often go down in value.
When a book shop is offering that much on a set that you would get about 90 bucks for, then it is either a personal purchase or it is on someone’s list. I have about 30 shops in Europe, Asia and North America with my want list as well, what the lowest grade I would accept and price I would pay.
Its likely on someones list, so if it where something I fell into, and having no interest at all in that set I would place it on Biblio.com. Lots of collectors surf that site as well as brokers. Put it on for 750 to test the water.
Well unless it means something to you.
But a warning, getting lucky once or a few times even on paper does not make it a good investment to attempt an extra income in. I have a wonderful collection, I know many others with great collections, but the only people I have ever met who could make a go at selling rare paper are those who inherit our collection. Never met a person who just invested and knocked it out of the park. It always starts as one person passion who like me got some great deals over a long time but also over spent on many more items.
If books are not your thing and you have a chance to nail such a profit, then sell it because they will likely only loose value just keeping them.
Just my thoughts,