Value Of Small 1800-S Coin Set ?

I think you're right to look at what similar coins have sold for, not what they're listed for. Also, if you plan on selling them via the bay, there's always going to be an element of chance: who sees it, who wants it at that moment in time, and so on.

I don't think you'd get as much if you sold them as a single lot. I'm not sure why you bought the coins as a "collection", but a buyer would have to have the same want list you had when you bought them. Second, when people look at lots, they expect a discount. Do you want to factor that in? Cost-wise, you'd save a little in fees if you sold them all at once, but personally I wouldn't think too much about that. Sellers always pay on the bay.

You wanted a number but as explained above you've already got a good estimate, but the resale depends on how you sell them. IMHO, of course.