These types of items are usually a simple resistor that replaces a certain sensor's feedback. For example, in motorcycling world, most of the new bikes have a timing retard built into them that usually only works in lower gears ( to comply with noise/pollution regulations), so a 'timing retard eliminator' or a TRE can be installed that takes care of that problem. The TRE is either a resistor or simply a short circuit to ground, depending on the bike/etc.

One of the feedbacks on this guy's ebay account says 'I can't find the oxygen sensor'... this kinda tells me that this product perhaps replaces the oxygen sensor's resistance with a value that shows either too much oxygen (causing ECU to add more gas to the mixture to recover), or too little oxygen (causing ECU to reduce the gas to mixture) ... niether of the situations are desirable in the long run.

Also, when someone's english is as poor as this seller's, I usually start to doubt their technical knowledge. (Anyone detailed enough to come up with a GOOD product, will be detailed enough to make sure their ebay ad is worded properly.)

I would pass on this if I were you.