I'm not much of an IMer here, but I can give you my opinion, as I have bought and sold with SH. In my experiences, it has been better to buy on eBay and sell on SH. SH fees are pretty enormous either way, but people are usually willing to overpay on tickets and they usually cover the seller fees to make it worth it. If you are going to buy, it is really only worth it if the tickets you are buying are expensive. It just doesn't work for $20 tickets, as the buyer fees are not that much different than those of pricier tickets.
Even though I complain a little, it is the online ticket service I use most, when tickets for an event are sold out. They are an extremely secure service, so I would recommend them.