There are currently some interesting price (or at least display) discrepancies within coinbase itself.

For example, when it displays the worth of my bitcoins in USD, it is valuing them at about $107, but the buy/sell prices are around $102.

I'd probably prefer to use craigslist for either side of a sale like this.

Ebay is pretty safe for the buyer but the prices tend to reflect that. Also difficult to get a good price at auction. On craigslist you are more likely to get a good deal from someone who doesn't price their stuff correctly, though you have to hustle.  perhaps I'm ust too nervous,but I would prefer craigslist as a seller as well. Meet in neutral place, get cash immediately, no worries about chargebacks, etc.

I just bought a couple of coins there recently as well. Does anyone know the reasons behind this discrepancy?

Your best bet is probably to check ebay for similar models, there are always a lot of used MBPs for sale there.