I'm planning for someone who isn't going to be able to get proper luggage before going to the airport. They're going to be leaving from SFO, and want to know if they can buy luggage there to use for the trip. It looks like there's a bunch of Pacific Outfitters where you can buy luggage but they're all post-security. Would there be any issues with going through security with something like a tied up shopping bag, buying some luggage in there, and then going out to check those bags in? Would there be a better alternative?
I believe that you are right that all the stores that sell luggage at SFO are post-security. They aren't going to have a particularly large selection (possibly just carry-on sizes only), and this traveler would surely be better off stopping at a real luggage store on their way to the airport if at all possible, but I'll accept that's impossible for them.
I believe it's quite likely that they would be able to go through security with a shopping bag or something similar. It's possible they'll have a problem with a large volume of makeshift bags though. If there's more than one person traveling, the solution is easy: one person can go through security, buy the bags, then come back out. There's no problem going through security more than once at SFO.
One potential issue is that Pacific Outfitters has locations in Terminal 2, and both the G and A sides of the international terminal, but not in Terminal 1. Terminal 3 is linked post-security with the G gates, so it's easy to get over there from either side, but there's no connection to Terminal 1. If this person is flying out of Terminal 1, they won't have access to the shop (it's possible they could go through security in Terminal 2 or the international terminal, come back out, and then go to their terminal, but that's contingent on security letting you go through in a terminal where your flight doesn't leave, which they likely aren't supposed to do). TLDR: they may have an issue if they are flying out of Terminal 1.
One other option would be to contact the Airport Travel Agency at SFO. They say they offer "Cargo Bags for carry-on or checking." You'd have to check with them to see exactly what those are and how much they cost. Their office is pre-security in the international terminal.
A final option would be to try to secure a very sturdy cardboard box before headed to the airport. If they're staying in a hotel, the hotel may be able to help (I just did this last week due to an emergency suitcase malfunction), or they could try a nearby local shipping store. Airlines do not consider a box to be proper checked luggage, and they will have to sign a liability waiver to check it, but it can get the job done in a pinch. Note that boxes will take a good beating in the cargo hold from being pressed up against suitcase wheels, punching holes in the side, so you absolutely want a sturdy box and to put any loose items in a plastic bag inside the box.
He/she can also buy one of these super folded travel bags, have it folded at wallet size before going to airport, and then use it after security as travel cabin bag. Usually Duty Free also sell this type of bag plus normal travel bags but I am not sure for this airport, you can email the airport direct maybe?