It's been a long time since I shipped a motherboard, but they used to have some Priority Mail boxes that fit a full ATX board pretty well. They give those boxes away for free but you have to use them for Priority Mail. Your local post office will give them out, or you can browse the available boxes on the USPS web site and order them in sets of 25 or something like that. They'll deliver them free, grand total $0 but it takes a couple weeks for them to show up. Smaller motherboards can ship in those boxes for under 2lbs, larger boards will go over 2lbs so they cost more.
There's a size of "flat rate box" that also fits motherboards and is a bit bigger. If the package weighed over 2lbs and was going halfway across the country then it was cheaper to use the flat rate box, otherwise variable rate was cheaper. That was several years ago though, and USPS has changed their shipping rates multiple times since then, and probably changed some of the boxes also. Nowadays I think you're looking at over $10 to ship a <2lbs box via Priority Mail, and more than that for >2lbs or flat rate.
If you have retail motherboard boxes, you can turn those inside out and use that as a shipping box. It doesn't leave as much room for bubble wrap as one would like, but it's fairly strong and gets the job done without having to buy boxes. Using your own box means you can use the Parcel shipping service or whatever they call it now, but I'm not sure it's much cheaper than Priority now.
For best protection, not only does a box need to withstand a fall but it also needs to withstand something heavier falling on top of it. This can be an issue with light items in bubble mailers. The strength of the free USPS Priority Mail boxes seems a bit marginal, but they get by. I don't recall ever getting a complaint of them being smashed, though I'm sure they're more prone to it than most boxes are.
I've sometimes seen people ship items in soft packaging with lots of bubble wrap. Bubble wrap is not an effective defense against crushing. It slightly spreads out the load but it's not enough against a major blow. A heavy blow against bubble wrap will be transmitted straight to the item inside.
As a buyer, my attitude about packaging is that if it's a cheap item then I'd like to have the option of paying less for shipping and accepting the risk of weak packaging. I don't mind getting a $5 card in a bubble mailer if it means I can pay less for shipping. If a surface mounted cap or inductor gets popped off I'll tack it back on. If a fan gets damaged it probably needed to be replaced anyway. There are some items that aren't worth the cost of shipping safely.
However, proper boxed packaging should certainly be the default unless a buyer has clearly agreed to take the risk of going cheap. While I'm sometimes in favor of cheap packaging, that's only if I've agreed to it, I don't like being surprised by it.
*Realistically* - especially if you go on eBay - I'd be very careful about offering the "cheap and risky" shipping option because some people might get angry about it even if the packaging method was clearly explained. Especially in the anonymous world of eBay - some eBayers are quite immature (speaking both of buyers and sellers) and you don't know what kind of person you're dealing with. The less drama you have on there the happier you'll be. In the world of forums you might get a better chance to communicate with people before selling to them.
SIMMs/DIMMs: I've always shipped these in shielded bags padded with some extra cardboard padding and shipped in a bubble mailer. They're plenty safe, they don't have anything sticking out that will break off and it would be very hard to crush them.
The main reason for the extra cardboard was because of a weird detail in the USPS rules. You couldn't get tracking unless the package was considered a "parcel", and it wouldn't be considered a "parcel" unless it exceeded some minimum thickness. I don't know if that rule is still the same. I was worried that if I put a parcel label with tracking on a package that was too thin, somebody inside USPS could decide to block it.
BTW - don't trust local USPS employees to know the finer details of their rules very well. They're quite good at believing and repeating things that they haven't actually researched, and when you research them yourself you may find something different.
