Terry, I continue to be jealous of your finds. Nice job on that Potter lot. My most recent CL purchase, 7228 Batman I Batwing, has provided some insight to the Used set game (on an aside, FCB has provided me tremendous help on this).
First, make sure ALL the figs are there, and they are all complete and in good shape. I had to buy a vintage Batman I fig to complete 7782 set, and that little @sswipe was $20 plus shipping. I had budgeted $15 - on a set I hope to sell for $120-130, every $5 helps.
Also, to echo tony - make sure all the stickers are either applied (to the correct bricks) and in good shape, or on the sticker sheet. That same set had really old, beat up stickers (with a few missing), forcing me to buy a sticker chart - another $20 down the crapper. Make sure your accessories are there as well - the Joker's henchman has 2 guns that were missing, and those things cost $6 combined.
Make sure you use Bricklink to the best of its ability, by using the My Wanted List. There's a feature called "My Wanted List by Store" that will allow you to source your My Wanted List parts by store, saving tremendously on shipping costs - another underestimated cost I ran into with the Batwing lot. Shipping by lot will run you a minimum of around $2 per order, so if you have to place 5 orders, you're looking at a minimum of $10, and sometimes a lot more if you're ordering rare pieces from overseas.
Finally, make sure the instructions are there. The ones you describe shouldn't be that expensive, but they add up with everything else. Good luck, and great job on the find! I really enjoy the work that goes into flipping these used lots even though it can be time consuming.