My first comment is - don't be too paranoid about people stealing your photos. It is highly unlikely that someone would ask to buy your photos and then go ahead and steal them. If you use a site like smugmug, you can make galleries private with a password access, but I don't think you can do that with most of the free sites. You could of course, just send then some jpegs so they can see what you have.
I would send them jpegs and ask them what they are prepared to pay and what they want to use them for. That way you can decide if it is acceptable to you and negotiate if you see the need. You can hardly have a standard price if you have never sold any before.

Some more comments on privacy. I post most of my photos as full screen size. I started doing this because I didn't think anyone would buy them anyway. Then, they started too and it just keeps growing. It seems that most people are much more impressed by a high res image, than a tiny, watermarked version that doesn't portray the photo very well. I've now sold to at least 12 different countries, including Nat Geo in 3 European countries and 3 separate magazines in China, and all of this has been generated by good quality images posted on the internet. If I hadn't posted them, I wouldn't have sold anything. If you search on fungi or mushrooms, you will find my images, some of which are "stolen", ie they are not credited to me. I would prefer it if they were, but I am not going to lose any sleep over it, because it really doesn't matter that much. Very few companies who want to use photos will steal them. It isn't worth it, so they will email me and ask a price. I usually ask them to propose a price and we go from there. I usually send them high res versions and maybe some text and the deal is done. It is remarkably easy and allows me to spend most of my time taking photos rather than protecting my patch and negotiating deals.