A recent question on the Bogleheads forum was Exact sequence of an ETF TLH transaction:
Is there a recommended method of doing a TLH? I tend to like to TLH on market downturns if at all possible. Currently I start with the ETF that I would like to sell. I look at the bid/ask, and I generally choose the higher “ask” price to set as my limit order. Even though I’ve chosen a down day to sell this ETF, obviously I would like to get on the higher side of the bid/ask spread since I am selling. The moment the sale goes through I start working on buying the replacement ETF. At this point I do not like to miss the opportunity to complete the sale due to greed. I put my limit order at exactly the lower “bid” (not the higher ask) price. I observe it closely for a few minutes to half an hour to see what it’s doing. If it looks like it’s veering upwards I will raise my limit to the current bid price. If after a while it looks like I’m going to miss the boat, I simply raise my limit order to the ask price instead of the bid. Generally this will execute the order. Most of the time I execute both orders within a few minutes to half an hour without being hosed too serverely.
I thought it would be interesting to hear what people have to say. Anyway, the following is what I do:
I enter the two orders first, but with out-of-range prices using limit orders. I open two windows — one for the purchase, one for the sale.
At the point when I want to execute the TLH, I modify one of the existing order to the price which would complete the transaction. As soon as I verify that the order is completed, I modify the price of the second order (modifying an order’s price is much faster than entering the complete order).
Depending on the ETFs, sometimes I do the purchase first and sometimes I do the sale first. Generally, I like to complete the transaction with the lower liquidity / volume ETF first.
My preference for completing the transaction on the ETF with lower liquidity / volume first is because if I had done it the other way round, I might be unable to complete the second transaction at a reasonable trading transaction cost (due to undesirable bid-ask spread).