<p>I have a spool of PLA filament where the diameter is visibly inconsistent. So it will only print a meter or two and then start slipping instead of being fed into the nozzle. Is this still useful for anything or can it be salvaged somehow?</p>
<p>I can't return it, the shipping costs more than the spool.</p>

In principle you can re-extrude it with a somewhat simpler machine/setup than making filament from scratch, but controlling the diameter is the hard part of making filament - as you can see from how the manufacturer of yours botched it.
I would first try insisting on a refund without returning the item unless they pay return shipping, and that they cover the original shipping cost. The product they delivered is not usable for the purpose it was advertised for.
As for salvaging it - if that's what you really want to do, or if you end up stuck with it - as long as it's not too wide to fit through the filament path to the nozzle, an extruder that's spring loaded can
probably
manage to push it reliably. You will of course have pulsating under- and over-extrusion which will make it largely unusable for serious parts that need to be dimensionally accurate, strong, or visually appealing, but there are lots of things without these requirements it might be useful for. If your printer isn't capable of handling it, you could perhaps sell or trade it to someone whose printer can handle it.
Getting more on the wild end of things, there are filament diameter sensors that can be integrated with your printer and firmware (I'm pretty sure Klipper supports this; not sure about Marlin and others) to measure the diameter and compensate extruder motor steps to keep the extruded volume per requested E-axis length constant. This would in theory make it possible to use the bad filament for serious prints.