Alright, here we go:
All prices in USD. Sources: Gamestop.com; this Console Heroes thread [http://www.consoleheroes.com/forums/showthread.php?t=15170], Ebay.com (seller fees page [http://pages.ebay.com/help/sell/fees.html]). For convenience' sake, tax is assumed to be flat 10%, and undefined shipping costs are assumed to be $2.50 (approx. the rate of US to US priority mail) as a cost to the seller. Flat rate shipping is assumed to be neutral to both parties (i.e. paid in full to the postal office), and thus is simply a cost added to a buyer. Prices were pulled off of auctions that ended today (Oct. 21, 2009) within an hour or so from when this post was made. Rounding was made against the consumer (towards Ebay or Gamestop), except 0.99 is rounded to 1 for convenience' sake. Finally, be aware that the conclusive numbers are recorded as "Convenience Tax" or extra money you pay to shop or sell at Gamestop over Ebay. Thus, higher numbers favor Ebay over Gamestop, and lower numbers the reverse.
Halo 3 ODST
Gamestop: Buy From: $50 ($55 after tax)
Sell to: $30 (store credit); $24 cash
Notes: Edge card bonus here is avoided for convenience' sake. Store credit isn't taxed, which essentially means it's worth the tax rate more than cash in store (which is the only place that you can use it). Further, if the seller intends to buy certain titles (listed here [http://www.gamestop.com/gs/specialty/tradeins/]), they can take advantage of the "Power Trade" offer that goes on. Thus I consider both "base store credit" at a value of $33, and "extended store credit" at a value of (30*1.25)*1.1 = $41.25. (The current environment only serves as an approximation; this number changes constantly, but represents a theoretical maximum you can stretch your dollar.)
Ebay: Auction 1 [http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270471147827]
Winning Bid: $45 + $4 shipping
Ebay fees: $3.17 to $4.07 = 2.19 + (45-20)*3.5% + (flat fee from $0.10 to $1)
Buyer Convenience Tax: $6
Seller Convenience Tax: Cash: $17.83 to $16.93 = $21 - (3.17 to 4.07)
- Base Store Credit: $8.73 to $7.83 = $12 - (3.17 to 4.07)
- Extended Store Credit: $0.58 to -$0.32 = $3.75 - 3.17
Auction 2 [http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170394594874]
Winning Bid: $42 + $4 shipping
Ebay fees: $3.06 to $3.96 = 2.19+ (42-20)*3.5% + (0.10 to 1)
Buyer Convenience Tax: $9
Seller Convenience Tax: Cash: $15.04 = $18 - (3.06 to 3.96)
- Base Store Credit: $5.94 to $5.04 = $9 - (3.06 to 3.96)
- Extended Store Credit: -$2.31 to - $3.21 = 0.75 - (3.06 to 3.96)
I had a third auction [http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270472061680] lined up but it turns out that someone paid $41+$4 shipping for the campaign disc half of the game. I have a sinking feeling that guy got ripped off.
Buy It Now Example [http://cgi.ebay.com/Halo-3-ODST-Xbox-360_W0QQitemZ170397163570QQcmdZViewItemQQ_pidZ56270292QQmediaitemZ1QQptZVideo_Games_Games?hash=item27ac766032]
Cost: $38 + $4 shipping
Ebay Fees: $5.85 = 0.15 + 38*15% (Fixed price listings have much less variable fees)
Buyer Convenience Tax: $13
Seller Convenience Tax: Cash: $8.15 = 14 - 5.85
- Base Store Credit: -$0.85 = 5 - 5.85
- Extended Store Credit: -$9.10 = -3.25 - 5.85
It should be noted that store credit is very much irrelevant in this scenario; the Buy it Now option is most likely to be used by merchants rather than individuals, who are out to make profit rather than out to get the new best game (making the Extended Store Credit model to be doubly irrelevant). They are included here for completeness.
As a seller, if cash is the goal, then it is always better to pursue Ebay above Gamestop. Even with base store credit, it is also generally better to pursue Ebay, although the convenience tax of $5-6 is not out of the question, especially considering it could be several days or even weeks before you see your money. Seeing as Ebay has no good ways of marketing a product, the final value of an auction is highly variable. While not observed, it is entirely possible to end up with the short end of the stick, selling-wise. Finally, all of Gamestop's offers are taken advantage of, which means the seller is shrewd about when to sell, it is surprisingly possible for Gamestop to come out on top, which was somewhat unexpected.
Note well: with a sample size this small, it is easy to be in error. Furthermore, this is one title and it may be in a unique position compared with other titles; for instance, being a new, in-demand title, its comparative prices may be higher than average. More research is clearly needed. If anything is in error or is misrepresented, feel free to point it out.
Brief Second Opinion
Grand Theft Auto IV (XB360)
Gamestop: Buy from: $25 (27.50)
Sell to: $9 (store credit); $7.2 (cash)
Base Store Credit: $9.90
Extended Store Credit: $12.37
Ebay: Auction 1 [http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=230388338849]
Winning Bid: $16.49 + $4 shipping
Ebay fees: $1.55 to $1.80 = 16.49 * 8.75% + (0.10 to 0.35)
Buyer Convenience Tax: $7.01
Seller Convenience Tax: Cash: $7.74 to $7.49 = 9.29 - (1.55 to 1.80)
- Base Store Credit: $5.04 to $4.79 = 6.59 - (1.55 to 1.80)
- Extended Store Credit: $2.57 to 2.32 = 4.12 - (1.55 to 1.80)
Despite being over a year and half old, GTA IV puts up numbers comparable to ODST, and does nothing to refute the initial conclusions.