I've put together several group purchases much like you describe. The first one was for six or seven SAR-1s, along with an FAL. We got these just before they were banned here in California. I contacted several friends who were at least somewhat interested in guns (some much more than others). I knew them all pretty well, except for one guy who was sort of "a friend of a friend." It turned out fine. I've since put together couple of additional group purchases of Bulgarian Makarovs and PA-63s. Some of these saved me a little money (slightly lower prices and shipping costs), but not a lot over what I'd have paid to just get my own. But I enjoyed helping friends get good deals on guns they never even knew they wanted until I pointed out what good deals they were
I've found that it is a lot easier to get people interested if (1) there is a law about to ban the gun, and (2) it is a very good deal (Maks for $105, PA-63s for $90).
You did hit on some of the risks. I have always put the guns on my own credit card and paid them off right away, but sometimes it took a few months to get the money from the buyers (not that they were trying to rip me off - they just didn't get around to paying for awhile - sometimes because it took me awhile to figure out and tell them the total cost). If this is a big issue for you, I'd suggest getting the money up front. In my case, at least, holding the gun hostage probably wouldn't have mattered, since some of they guys weren't in any hurry to pick them up anyway.
As far as distributing the guns (which may vary in condition or quality of construction), I have always done this randomly. It is tempting to say that I get first pick, since I'm doing most of the work and paying for them, but I haven't done this. I usually just tell my FFL to assign the guns randomly by serial number to each buyer (without actually looking at the guns). No one has questioned this, but it might be cleaner to have some formula like "lowest serial number to youngest guy" or to draw serial numbers out of a hat or something. I think if I'd let people pick their guns, it would have been sure to create hard feelings.
I do usually tell everything I know about the guns and costs to the buyers by e-mail before the fact (so there are no misunderstandings). I usually throw in a disclaimer to the effect that if we get ripped off somehow (guns not right, costs higher than expected, etc.), we are all agreeing to share the cost. I don't guarantee anything personally except that I'm being honest and taking the same risk they are. I also say that once the guns are randomly distributed, each person is responsible for working out any problems with his particular gun with the vendor or importer (of course, I'd help if I could, but it is ultimately their problem). I tell them approximately what the FFL and related charges will be (usually padded a little so that the actual cost turns out to be a dollar or two less than I'd estimated), and have them pay the FFL directly when they do the paperwork on their guns. They reimburse me for my out-of-pocket cost. So far, no one has ripped me off, or had any real complaints about these deals (at least that I'm aware of).
Finally, I've ordered most of these through Aim Surplus, and been very happy with their service. They did mess up once and sent us one SAR-2 or SAR-3 instead of a SAR-1, but they were very easy to work with and willing to make it right. I've also used J&G Sales a time or two.
I think these group purchases are a good way to go IF you save some significant money AND you think it would be fun to play gun salesman to some of your friends (as I do). I don't think it would be worth the headaches if you are only doing it to save $20. Good luck.
Doug