Know the make/model of your computer.
Use the part number & other information printed on your battery: I've seen some used computers that didn't have the correct battery though.
--- So double check the battery compartment as that should have the exact battery information for the computer in use.
One on-line company I deal with is laptopbatteryexpress
They state Should I buy a used Laptop Battery? All our laptop batteries are brand new, hand-tested and shipped from the USA. We use only the highest quality lithium-ion cells and components. We're so confident that we back each product with a 2 year warranty and 30 day money back guarantee.
--- The 3 sources already mentioned are fine too as I have dealt with them also.
Naturally the cost difference of used batteries might be significant but I don't ever check for used or refurbished batteries so I don't know what the differences are.
On the place you were at, if they are a reliable company and have quality batteries, consider bringing your laptop and battery you want to replace. If the price is decent compared to what you see on-line, try the battery recommended by that company right away