I am not sure there really is a problem, on the whole what we have now works well.
This is life, no guarentees, no contracts, no certainty. The seller can sell to who he wishes, at whatever price he wants, end of.
If someone asks for details, more info, thats interest. How the seller interprets this is up to them, and communication here can help.
Saying dibs means jack shit except a strong interest that you want to buy. Saying I'll take it means that it would be hard for seller to sell elsewhere, and to me would make it hard to back out of on either side without a kick in the bollox.
If you really want something, reply quickly and agree to buy it. If this is without pics, thats your risk then. If you want more info, then its your risk that it may get taken in the meantime. Sellers discretion.
Its happened to me where some unlikely scrote
picked up a bargain that I wanted as they could respond quicker, hey ho. Part of the fun really
-- note to be clear tongue firmly in cheek on the scrote comment --
A bit of gentlemanly conduct and good comms should sort out most issues.