Why does Trojan copy sells but not buys?

Trojan's docs say wallets that buy through custom programs cannot be copied. Check the buy on Solscan for "Interact with System Program".

Do I need SOL even when copying stablecoin swaps?

Yes. Trojan's FAQ says you need SOL in the wallet for gas.

Fund the execution wallet. Copy trades run from the wallet you chose and it needs SOL for network fees. Trojan's FAQ says you need SOL for gas even when swapping stablecoins.
Check the target wallet can be copied.
- Wallets that buy through custom programs cannot be copied. Open a buy on Solscan in legacy mode: if it shows "Interact with System Program", the wallet cannot be copied. This is a likely cause if only sells are copied.
- Not all pools are supported. Confirm on Solscan that the target trades in supported pools.
- Wallets that buy in the first seconds of a launch are poor targets because you enter after them.
Slippage. If trades fail on slippage, Trojan's FAQ says to raise it. Copy trades usually need more than manual trades. See what slippage is.
Priority fee, bribe and MEV protection. Trojan's guide says to set these up, for selling as well as buying.
Sell settings. The three options are Copy Sell, Auto Sell (take profit and stop loss) or neither. When editing a profile, switch from Buy to Sell and fill in both.
Alerts. Add alerts for the target's trades to confirm activity is detected.

If nothing helps, send Trojan support the wallet addresses, transaction links and error messages. See Trojan and the Telegram bots page.