Sega Genesis is a nice system to collect for, but there are many available in the marketplace. Be aware that there were actually about 7-8 variants of the console released. The earliest consoles have "High Definition Graphics" boldly printed on top, and most importantly, do not have TMSS, a routine Sega later implemented for verifying that any game cart was "officially licensed" by Sega for the system. These consoles tend to be the more highly valued among Genesis systems because they play even the earliest 3rd party games from EA. TMSS systems will not play some early EA games, so be forewarned.
Next, there is another "model I" system that still has the "High Definition Graphics" logo on top, but does have TMSS in the system BIOS. The way you will know it has TMSS is to start the system with a game inserted. If it immediately goes to the game's start screen then it does not have TMSS. If the system first takes you to a screen where you see "Produced By or Under License From Sega Enterprises LTD.", then it has TMSS. This is less desirable among collectors.
Then there is the model I console that does not have "High Definition Graphics" across the top of the console. It has TMSS.
Next are the model II variants. These are known by their slimmer and more aerodynamic styling. Turn these consoles over and look through the vent slots and you will see a tinned RF shield. Some model II's had a full motherboard that went all the way across the vent slots while others had the 3/4 motherboard that only goes partly across the vent slots. Some were made in Taiwan, some in Japan, and others in China. It gets muddled, but to hardcore Genesis collectors, there is an optimal motherboard in these model II's that had the clearest video and sound outputs. My guess is that the average buyer on ebay has no knowledge of this though. For better details of the motherboard differences, see here
Finally, there is the smallest of the Genesis consoles, the model III, released in the later 1990's by Majesco under license from Sega. These are somewhat sought after because of their smaller footprint. They simply take up less shelf space on your entertainment center. Only thing to be warned about on the model II's is that they are not compatible with tghe Sega CD or 32X peripheral add-ons. Also, there is something different about the CPU chip in these that does not allow them to play Virtua Racing and a couple others.
Hope that helps.