The trick to avoiding mistakes like mine are very simple. As with birders, who tend to see a common bird without their guide book then decide it is some rare species later with book in hand, I advise you to write down the facts right away. Where the birder might write "Robin-sized bird with white chest, yellow pointed bill and brown streaked upper body" you should write down the details as well. Heck, in this day and age you have all sorts of options! Here is what I recommend whether it be for resale, a gift for a friend, or, an item for your own collection:
1. If you want to look something up later, write down the details. This should include color, materials, size, any and all markings, dates, tags, etc. on an item along with condition. When I am doing this right, I come home, look things up on the internet and 9 times out of ten, discover I made the right choice to leave it there.
2. Photo! If you have a camera, even a phone camera, take a photo.
3. Research while there. If you have a hand-held device, look it up on the spot! ( I now see book dealers standing in the Salvation Army diligently researching every single book on the shelves. If they can do it you can do it!)
4. Call a friend! If you have a friend or family member who will not roll their eyes when you call, have them do a search for you. This may be the best method if you are someplace you cannot return to later like a flea market or yard sale. I've even photographed items then emailed them to a friend from my phone for info.
5. Get the follow-up-facts. Take with you the information for how to get back in touch with the seller. Name, phone number, days/hours of business and also ask if they ship to you. I have done this with items I regretting not buying while on vacation. I called and a week later it was in my hands!
These things may be simple, but, doing any one or more of them can keep you from the obsession, waste of time and money that comes from walking away from an item when you have questions. I have on many occasions come home and felt relieved to have left items because they had not been the treasures I'd hoped. I have also been able to determine if an item is desired by a family member or not. I have on rare occasions, gone zooming back to a location with good reason! But, 9 times out of 10 your instinct to not purchase something will be the right one. Whatever you do, try to avoid what I did yesterday. My penance? I bought the book and will read "Of Human Bondage."