pre wound bobbins
I use them all the time in several of my machines. With most of them you just take the cardboard off and put the bobbin in your machine or bobbin case.
You have to make sure that you don't get the ones that are strictly for embroidery and are a very thin thread. Also that you have the right size, there are size L which fits most machines that take the smaller bobbin and the M for the machines that take a class 15 bobbin.
Saves a lot of time not having to wind bobbins, as the prewound have usually anywhere from 80+ yards to over 100 yards of thread on them.
I too bought mine on EBay and watched for sales on them and have many basic colors. I have not had any problems with them being weak thread.