On a seperate note, I watched a well informed guy at my shop last weekend tell someone their Molten Tail Masticore was worth $8.00. Made me so mad; that is stealing. If the owner of the Masticore said "I value this at $8", that's one thing, but lying to someone about the value of their cards is stealing. That's where I draw the line. If you tell them "I value it at $8, but it's worth $15 – $20", then they at least know and can choose, but straight up lying to them and telling them it's worth $8 is stealing.

Do readers agree?

I second ChuckP's comment. you have to convert my half of the trade to ebay values too.

2 Time Shifted Lord of Atlantis (2×4=8) 8.00
1 Power Artifact (25) 11.00
1 Force of Will (50) 40.00
3 Mint Lim Dul’s Vault (3×8=24) 10.50
5 Spell Snare (5×4=20) 17.50
1 MP Lim Dul’s Vault (6) 6.00
2 Elvish Spirit Guide (2×5=10) 10.00
1 Antiquities Hurkyl’s Recall (12) 12.00
1 Revised Demonic Tutor (10) 8.00
1 Booster box Scars of Mirrodin* (90) 90.00

Actual value: $213

I feel that with most players, even those who would describe themselves as trading-savvy, it will be difficult for them to do a 10 cards for 1 trade. that said, there is no harm in attempting to make that trade, and you may be able to increase your profit margin by offering a $10 with 3 $1 cards thrown in, rather than 1 $15 card

There used to be two stores in my area within a 30 min drive of each other. One was only Standard the other had a decent sized Vintage community. There was a guy who used to regularly go between the two shops and pick up staples in the other format for cheep and trade them away at the other shop.

Here's a question for you. When you're flipping through a binder and the only way the trade is going to happen is to get 3-4 cards for one is there a way you aproach this trade? Do you just tell them that you're going to have to trade down and try to get some value that way, or do you try to feel them out to see if they would trade 10 $2 cards for your one $15 card?