I've purchased two Dell OEM batteries from TPSTech before. The battery life and longevity was identical to the original battery that came with the laptop. The packaging it comes in is also similar to what Dell ships it's spares for onsite warranty replacements.
As long as the part number of the replacement is the same as your existing battery, there will not be any issues. They seem to be authorised retailers for spares of a few major OEMs. I'd rather trust them than a 3rd party brand that is much more likely to underperform or die sooner.
