They sold the watch at a reasonable price .(过去时，是指买完了。
They sell watches at a reasonable price（现在时，指总是这样）
你自己选吧。你的问句表达得不太清楚。
They sell watches at a good price.
They sell the watch at a reasonable price .我不知道你应用的语境，不过我觉得用sell的过去式sold更合理
They sold the watch at a reasonable price
They sell watches at a reasonable price.
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