There are several reasons. First, since no one knows what the reserve is, they figure they'll have a shot at it. True, there are a lot of cheapskates out there that actually think they're going to get the item for $5.00. Second, a lot of people don't understand the proxy system. So they'll nickel and dime the bids up, slowly nibbling away your reserve. That also will increase the chances of a successful sale.
Second, you have zero feedback. You have no prior history. And you're out of the country. And on top of that, you have NO picture. So who knows if you're a legitimate seller, or if you're trying to pull a fast one-especially with that kind of price.
As for changing the price on an existing auction, I don't think you can. I think you'll have to cancel the auction early and re-list it. But since there are no bids, you may be able to make the change. I don't know. I've never tried it. I try and set the price right when I open the auction.
And lastly, why try and take the lazy way out and lump them all together in one auction? What do you think the chances are that someone is going to be in need of that exact same set? You may have some models that one collector wants, but the rest, he already has. So to him, it makes no sense to bid on your item. And other models, another collector may want.
You will have far better odds of success running several "smaller" auctions as opposed to one big one.
If you are looking to get-say $700, why not run seven auctions, for $100 each? Put the planes together by type, or airline, or whatever. True, not all of them may sell, and you may only end up with $350. And you still have some models to try again later with. Isn't that better than nothing?
And who knows, you just might get lucky on a couple of auctions. You may do a "right" set, and the "right" collectors may start a bidding war. Who knows, you could conceivably end up with a lot more $$$ than you otherwise would've with the one auction.
But take my advice. Split the models into multiple auctions. Use a reserve, or your minimum "selling price". Don't do both.
And lastly, get some pictures.